Documentation cleanup

[sjpfenninger]

Summary of changes in this pull request:

• Moves over some of the most important docs pages from rst to md
• Removes all parts of the old documentation which have already been converted or are no longer needed, to make it easier to track what remains to go through

STILL TODO:

• building.md needs to be rewritten - formally it's now markdown, and I've made some initial changes to improve the structure and content, but especially in the latter parts it still refers to no longer existing functionality
• All newly-added docs need to be checked again for consistency with 0.7

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 (e2bfb1a) 95.66% compared to head (f19b1d9) 95.70%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #538      +/-   ##
==========================================
+ Coverage   95.66%   95.70%   +0.04%     
==========================================
  Files          24       24              
  Lines        3526     3562      +36     
  Branches      765      710      -55     
==========================================
+ Hits         3373     3409      +36     
  Misses         87       87              
  Partials       66       66              

Files	Coverage Δ
src/calliope/backend/backend_model.py	97.84% <100.00%> (+0.10%)	⬆️
src/calliope/backend/pyomo_backend_model.py	94.94% <100.00%> (+0.35%)	⬆️
src/calliope/cli.py	81.67% <100.00%> (ø)
src/calliope/examples.py	100.00% <100.00%> (ø)
src/calliope/model.py	93.65% <ø> (ø)
src/calliope/preprocess/model_data.py	100.00% <ø> (ø)
src/calliope/util/logging.py	93.87% <ø> (ø)

[brynpickering]

I expect the markdown link check to fail because there's a hyperlink to a page in the docs from CONTRIBUTING.md that will only exist once this is merged in and v0.7.0 is released.

EDIT: updated to not point to a specific page int he docs. This seems a better approach for longer-term maintenance.