Revise documentation

[gerlero]

As part of openjournals/joss-reviews#7633, make the following proposed changes to the documentation.

From #316 (comment):

• DOCS: describe how Slurm-based execution works with AsyncSlurmFoamCase.
• DOCS: mention that the FoamFile class supports regular expressions, include directives, and other #-based expansions; but does not itself expand these expressions.
• DOCS: mention that FoamFile does not (currently) support #codeStreams.
• DOCS: mention that TimeDirectory cell center computation only works with reconstructed meshes.
• DOCS: mention that non-FoamFile output files are also not supported by the FoamFile class.

From #317 (comment):

• DOCS: add a realistic, self-contained, use case example.
• DOCS: add code snippets from README to the relevant docstrings.
• DOCS: add more elaborate docstrings for main user-facing classes.

From #317 (comment):

• DOCS: improve documentation following the Documentation section of the review guidelines (consider extracting text from the paper).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.07%. Comparing base (9627c72) to head (abfc35d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #326   +/-   ##
=======================================
  Coverage   87.07%   87.07%           
=======================================
  Files          15       15           
  Lines        1408     1408           
=======================================
  Hits         1226     1226           
  Misses        182      182           

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