Improve developers' documentation

[jowezarek]

Help improving Psydac's docstrings - any contribution is welcome @pyccel/psydac-dev !

Files updated so far in this PR:

• ...
• ...
• ...

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To do

• Explain how to add a new file the documentation
• Explain what to do if an existing file has been renamed
• Point to numpydoc documentation for guidelines on docstring formatting
• Mention PEP-8 for conventions on general formatting
• Add workflows for compliance to numpydoc guidelines and PEP-8 conventions (see Pyccel)

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00%	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (ae3e803)	30704	18443	60.07%
Head commit (f1f5ef6)	61408 (+30704)	36886 (+18443)	60.07% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#390)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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