README.md: Update CI badges.

- Fix URL for the unix badge.
- Add stm32 CI badge.
- Add docs CI badge (linking to the documentation)
- Make docs CI run on push (so we get a badge generated).

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

.github/workflows/docs.yml

@@ -1,6 +1,7 @@
 name: Build docs
 
 on:
+  push:
   pull_request:
     paths:
       - docs/**