Add documentation for the math syntax

[brynpickering]

Fixes issue(s) #423

Summary of changes in this pull request:

• New section for inbuilt math documentation, given in long-form LaTex math syntax
• New section for "custom" math, including component syntax documentation and usage examples
• Rendered schema in the references section.
• Clean-up of docs more generally:
  • move all generation scripts to become hooks. Docs still currently build <10 seconds even when all generation scripts (incl. math docs) are hooks. Benefit is that they follow a consistent syntax and we can remove one dependency (gen-files).
  • Fix docstrings using old RST format and add some docstrings for methods included in the API docs.
  • Split API docs into their own pages so they are easier to manage in the nav/ToC.
  • Split example custom math into their own pages, again so they are easier to manage in the nav/ToC.
  • Simplify method of accessing calliope version dynamically by leveraging PyYAML's python/name syntax.

Reviewer checklist:

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3680cdc) 95.00% compared to head (6e8fb49) 95.18%.
Report is 2 commits behind head on main.

❗ Current head 6e8fb49 differs from pull request most recent head c7f7e40. Consider uploading reports for the commit c7f7e40 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #456      +/-   ##
==========================================
+ Coverage   95.00%   95.18%   +0.18%     
==========================================
  Files          24       24              
  Lines        3303     3305       +2     
  Branches      706      652      -54     
==========================================
+ Hits         3138     3146       +8     
+ Misses         96       92       -4     
+ Partials       69       67       -2     

Files	Coverage Δ
src/calliope/attrdict.py	96.48% <100.00%> (+0.03%)	⬆️
src/calliope/backend/backend_model.py	97.66% <100.00%> (+2.25%)	⬆️
src/calliope/backend/expression_parser.py	97.14% <ø> (ø)
src/calliope/backend/helper_functions.py	97.75% <ø> (ø)
src/calliope/core/model.py	94.70% <100.00%> (ø)
src/calliope/exceptions.py	94.87% <ø> (ø)
src/calliope/postprocess/postprocess.py	90.32% <100.00%> (ø)
src/calliope/preprocess/load.py	92.40% <100.00%> (ø)
src/calliope/preprocess/time.py	88.57% <100.00%> (ø)
src/calliope/util/logging.py	93.87% <100.00%> (+2.76%)	⬆️