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:

• [x] 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
• [x] All newly-added docs need to be checked again for consistency with 0.7

Reviewer checklist:

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

```diff @@ 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](https://app.codecov.io/gh/calliope-project/calliope/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project) | Coverage Δ | | |---|---|---| | [src/calliope/backend/backend\_model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvYmFja2VuZF9tb2RlbC5weQ==) | `97.84% <100.00%> (+0.10%)` | :arrow_up: | | [src/calliope/backend/pyomo\_backend\_model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvcHlvbW9fYmFja2VuZF9tb2RlbC5weQ==) | `94.94% <100.00%> (+0.35%)` | :arrow_up: | | [src/calliope/cli.py](https://app.codecov.io/gh/calliope-project/calliope/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2NsaS5weQ==) | `81.67% <100.00%> (ø)` | | | [src/calliope/examples.py](https://app.codecov.io/gh/calliope-project/calliope/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2V4YW1wbGVzLnB5) | `100.00% <100.00%> (ø)` | | | [src/calliope/model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL21vZGVsLnB5) | `93.65% <ø> (ø)` | | | [src/calliope/preprocess/model\_data.py](https://app.codecov.io/gh/calliope-project/calliope/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL3ByZXByb2Nlc3MvbW9kZWxfZGF0YS5weQ==) | `100.00% <ø> (ø)` | | | [src/calliope/util/logging.py](https://app.codecov.io/gh/calliope-project/calliope/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL3V0aWwvbG9nZ2luZy5weQ==) | `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.