Create readthedocs CI job for TF-PSA-Crypto

[gilles-peskine-arm]

Create a readthedocs job on the CI, similar to what we have on https://github.com/Mbed-TLS/mbedtls.

The immediate goal is to be able to see the rendered documentation when we review a pull request. This is particularly relevant for the Q2 work where we're making a lot of changes to which APIs are public vs private, and how APIs are marked as public vs private.