Misc fixes for doc

[TL231]

Some small fixes for workflow related issues.

1. Added doctest and coverage extension for workflow.
2. Renamed docs to doc since that's what the workflow looks for, let me know if this causes an issue.
3. Added _static folder and some static files from qibo repo, it's the logo and css to stop doctest from reporting missing static folder.

[codecov]

Codecov Report

Patch has no changes to coverable lines.

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[damarkian]

Getting this error on merge:

Configuration error:
This documentation is not using furo.css as the stylesheet. If you have set html_style in your conf.py file, remove it.
make: *** [Makefile:20: html] Error 2
make: Leaving directory '/home/runner/work/qibochem/qibochem/doc'
Error: Process completed with exit code 2.

[TL231]

Google search says it's a sphinx 6.x version error, sure enough, the sphinx version in our poetry is 6.2.1.
I'll test it in another branch since the doc building/testing workflow is currently only set to fire when merging into the main so need some workflow changes to test it.