Support GitHub alerts

[rphair]

CIP editors & reviewers — a bit naively with respect to cips.cardano.org — have approved & merged some CIPs that use the GitHub "Alerts" syntax (called other things, including "Admonitions", on other Markdown based renderers):

• GitHub Docs > Basic writing and formatting syntax > Alerts

On cips.cardano.org this syntax gets no special treatment vs. regular Markdown, and therefore is shown as regular blockquotes beginning with the alert keyword. The worst consequences of this are in CIP-0137 (cc @jpraynaud) whose "note" offset passages are longer than the usual admonition:

• https://cips.cardano.org/cip/CIP-0137
• (vs proper) https://github.com/cardano-foundation/CIPs/blob/master/CIP-0137/README.md

Though we have some more CIP/CPS drafts that use this syntax, the only other merged document using "alerts" is CIP-0089 (cc @fallen-icarus):

• https://cips.cardano.org/cip/CIP-0089
• (vs proper) https://github.com/cardano-foundation/CIPs/blob/master/CIP-0089/README.md

The CIP editors (cc @Ryun1 @Crypto2099), and the more progressive authors, would appreciate if you would add some input massage and/or styling that will make these parse properly & display somewhat like they would on GitHub.

[aldo555]

Support has been added!