Fixing warnings

[mohanad0mohamed]

Fixes https://github.com/google/gf180mcu-pdk/issues/20

Renamed code-of-conduct.rst and contributing.rst to code-of-conduct.md and contributing.md to remove their warnings of not included in any toctree.

• [x] Built locally successfully

[mohanad0mohamed]

That's why I renamed those files:

[mithro]

You can fix the warnings by adding the files to the exclude list in the conf.py see the skywater-pdk one for example.

[mohanad0mohamed]

Then no need for this pull request.