Add module docstrings

[sajith]

Resolves #235 partly. Changes:

• Adds module docstrings, with links to FABRIC glossary where it makes sense.
• Avoids duplicating constructor docstrings as class docstrings. (Sphinx allows a choice between "init", "class", or "both", and I chose "class", because I thought the docs generated that way looked nicer.)

Please see the PR build to see the rendered version: https://fabric-fablib--256.org.readthedocs.build/en/256/index.html

[coveralls]

Pull Request Test Coverage Report for Build 7628803139

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 22.239%

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Totals
Change from base Build 7628190048:	0.1%
Covered Lines:	1158
Relevant Lines:	4641

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💛 - Coveralls

[kthare10]

Looks good!