docs: Add dependencies for sphinx to 'Build docs' action.

[Josverl]

Fixes broken documentation build due to missing module.

The modules needed to build the documentation are listed in docs/requirements-docs.txt to have a single file for these.
(alternatively this could be a 'docs' section in a common pyproject.toml file)

Fixes: #12498
See : https://github.com/orgs/micropython/discussions/12456

[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS

[codecov]

Codecov Report

Merging #12503 (ebcaf53) into master (da193c4) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head ebcaf53 differs from pull request most recent head a93ebd0. Consider uploading reports for the commit a93ebd0 to get more accurate results

@@            Coverage Diff             @@
##           master   #12503      +/-   ##
==========================================
- Coverage   98.39%   98.39%   -0.01%     
==========================================
  Files         158      158              
  Lines       20939    20938       -1     
==========================================
- Hits        20602    20601       -1     
  Misses        337      337              

see 1 file with indirect coverage changes

[dpgeorge]

alternatively this could be a 'docs' section in a common pyproject.toml file

If that's possible I think that's a cleaner solution. Can you please try to do that?

[dpgeorge]

alternatively this could be a 'docs' section in a common pyproject.toml file

If that's possible I think that's a cleaner solution. Can you please try to do that?

As far as I can tell, it's not possible to use pyproject.toml to specify requirements for pip. So let's go with the approach in this PR.

[Josverl]

alternatively this could be a 'docs' section in a common pyproject.toml file

If that's possible I think that's a cleaner solution. Can you please try to do that?

As far as I can tell, it's not possible to use pyproject.toml to specify requirements for pip. So let's go with the approach in this PR.

I could only get this to work with poetry, not with hatchling. And that seems like a bit too much overhead for this