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/**

README.md

@@ -1,4 +1,4 @@
-[![CI badge](https://github.com/micropython/micropython/workflows/unix%20port/badge.svg)](https://github.com/micropython/micropython/actions?query=branch%3Amaster+event%3Apush) [![codecov](https://codecov.io/gh/micropython/micropython/branch/master/graph/badge.svg?token=I92PfD05sD)](https://codecov.io/gh/micropython/micropython)
+[![Unix CI badge](https://github.com/micropython/micropython/actions/workflows/ports_unix.yml/badge.svg)](https://github.com/micropython/micropython/actions?query=branch%3Amaster+event%3Apush) [![STM32 CI badge](https://github.com/micropython/micropython/actions/workflows/ports_stm32.yml/badge.svg)](https://github.com/micropython/micropython/actions?query=branch%3Amaster+event%3Apush) [![Docs CI badge](https://github.com/micropython/micropython/actions/workflows/docs.yml/badge.svg)](https://docs.micropython.org/) [![codecov](https://codecov.io/gh/micropython/micropython/branch/master/graph/badge.svg?token=I92PfD05sD)](https://codecov.io/gh/micropython/micropython)
 
 The MicroPython project
 =======================