[docs] Fix 301 links

Author: oliviertassinari

Diff for: docs/data/base/getting-started/support/support.md

@@ -120,4 +120,4 @@ and
 <a
   data-ga-event-category="support"
   data-ga-event-action="tidelift"
-  href="https://tidelift.com/solutions/schedule-demo?utm_source=npm-material-ui&utm_medium=referral&utm_campaign=enterprise">request a demo today</a>.
+  href="https://www.sonarsource.com/tidelift-request-demo/?utm_source=npm-material-ui&utm_medium=referral&utm_campaign=enterprise">request a demo today</a>.

Diff for: docs/data/joy/getting-started/support/support.md

@@ -120,4 +120,4 @@ and
 <a
   data-ga-event-category="support"
   data-ga-event-action="tidelift"
-  href="https://tidelift.com/solutions/schedule-demo?utm_source=npm-material-ui&utm_medium=referral&utm_campaign=enterprise">request a demo today</a>.
+  href="https://www.sonarsource.com/tidelift-request-demo/?utm_source=npm-material-ui&utm_medium=referral&utm_campaign=enterprise">request a demo today</a>.

Diff for: docs/data/material/customization/css-theme-variables/overview.md

@@ -6,7 +6,7 @@
 You can implement them to improve Material UI's theming and customization experience.
 
 :::info
-If this is your first time encountering CSS variables, you should check out [the MDN Web Docs on CSS custom properties](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties) before continuing here.
+If this is your first time encountering CSS variables, you should check out [the MDN Web Docs on CSS custom properties](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_cascading_variables/Using_CSS_custom_properties) before continuing here.
 :::
 
 ## Introduction

Diff for: docs/data/material/customization/how-to-customize/how-to-customize.md

@@ -92,7 +92,7 @@ Target the `.Mui-selected` global class name to customize the special state of t
 <MenuItem selected className="MenuItem">
 ```
 
-If you'd like to learn more about this topic, we recommend checking out [the MDN Web Docs on CSS Specificity](https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity).
+If you'd like to learn more about this topic, we recommend checking out [the MDN Web Docs on CSS Specificity](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_cascade/Specificity).
 
 #### Why do I need to increase specificity to override one component state?
 

Diff for: docs/data/material/getting-started/support/support.md

@@ -129,4 +129,4 @@ and
 <a
   data-ga-event-category="support"
   data-ga-event-action="tidelift"
-  href="https://tidelift.com/solutions/schedule-demo?utm_source=npm-material-ui&utm_medium=referral&utm_campaign=enterprise">request a demo today</a>.
+  href="https://www.sonarsource.com/tidelift-request-demo/?utm_source=npm-material-ui&utm_medium=referral&utm_campaign=enterprise">request a demo today</a>.

Diff for: docs/data/system/experimental-api/css-theme-variables/css-theme-variables.md

@@ -5,7 +5,7 @@
 [CSS variables](https://www.w3.org/TR/css-variables-1/) are a modern cross-browser feature that let you declare variables in CSS and reuse them in other properties.
 
 :::info
-If this is your first time encountering CSS variables, you should check out [the MDN Web Docs on CSS custom properties](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties) before continuing here.
+If this is your first time encountering CSS variables, you should check out [the MDN Web Docs on CSS custom properties](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_cascading_variables/Using_CSS_custom_properties) before continuing here.
 :::
 
 ## Introduction

Diff for: docs/data/system/getting-started/support/support.md

@@ -120,4 +120,4 @@ and
 <a
   data-ga-event-category="support"
   data-ga-event-action="tidelift"
-  href="https://tidelift.com/solutions/schedule-demo?utm_source=npm-material-ui&utm_medium=referral&utm_campaign=enterprise">request a demo today</a>.
+  href="https://www.sonarsource.com/tidelift-request-demo/?utm_source=npm-material-ui&utm_medium=referral&utm_campaign=enterprise">request a demo today</a>.

Diff for: docs/pages/blog/making-customizable-components.md

@@ -24,7 +24,7 @@ Feel free to skip this section and move on to [Logic modification](#logic-modifi
 ### Good old CSS
 
 Let's start with the easiest part: modifying the style.
-This will necessarily involve CSS—especially the notion of [specificity](https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity), which says that if an element is targeted by two CSS selectors, the browser will apply the more specific one.
+This will necessarily involve CSS—especially the notion of [specificity](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_cascade/Specificity), which says that if an element is targeted by two CSS selectors, the browser will apply the more specific one.
 Usually this means that the selector with more classes applied to it is more specific and therefore takes precedence.
 
 For example, if we look at the Material UI `Switch` component, we have multiple subcomponents that we could expect to modify.

Diff for: docs/pages/blog/mui-core-v5-migration-update.md

@@ -78,7 +78,7 @@ All prop descriptions are now written in TypeScript, which means that you can ac
 
 ### 5. Upcoming CSS variables support
 
-[CSS variables](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties) will help us solve a lot of customization issues in the future.
+[CSS variables](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_cascading_variables/Using_CSS_custom_properties) will help us solve a lot of customization issues in the future.
 One of them is the dark mode flashing issue that has been requested for a long time.
 
 More importantly, this feature is opt-in if you are using Material UI v5!