Documentation updates

[brynpickering]

Fixes #555

Changes made in this Pull Request

• rename "custom math" to "user-defined math" across the board.
• rename "inbuilt math" to "pre-defined math" across the board.
• 4-space -> 2-space indentation in all YAML snippets.
• Added graphic to help visualise how the math syntax gets converted to a matrix operation.
• Cleaned up some confusing text that wasn't correctly updated from old docs.
• Update latex math representation to improve readability (as per @irm-codebase's suggestion).

Reviewer checklist:

• [ ] Test(s) added to cover contribution
• [ ] 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 (fd2917b) 95.76% compared to head (39861c3) 95.76%.

:exclamation: Current head 39861c3 differs from pull request most recent head eb953af. Consider uploading reports for the commit eb953af to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #559 +/- ## ======================================= Coverage 95.76% 95.76% ======================================= Files 24 24 Lines 3566 3566 Branches 777 711 -66 ======================================= Hits 3415 3415 Misses 86 86 Partials 65 65 ``` | [Files](https://app.codecov.io/gh/calliope-project/calliope/pull/559?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/559?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvYmFja2VuZF9tb2RlbC5weQ==) | `97.87% <100.00%> (ø)` | | | [src/calliope/backend/helper\_functions.py](https://app.codecov.io/gh/calliope-project/calliope/pull/559?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvaGVscGVyX2Z1bmN0aW9ucy5weQ==) | `97.75% <ø> (ø)` | | | [src/calliope/backend/latex\_backend\_model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/559?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvbGF0ZXhfYmFja2VuZF9tb2RlbC5weQ==) | `96.24% <ø> (ø)` | | | [src/calliope/model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/559?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% <100.00%> (ø)` | |

[brynpickering]

@irm-codebase and @sjpfenninger, ready for review.

@irm-codebase, you can view the documentation with the PR changes by selecting "Details" of the readthedocs CI check (once it has a green tick mark next to it):

[irm-codebase]

Most things look great so far! I only found one funky detail in the schema:

[brynpickering]

Most things look great so far! I only found one funky detail in the schema:

Yeah, this is an issue with the upstream dependency. It can't handle "OneOf"/"AnyOf"/"AllOf" schema elements. Another reason perhaps to have a different place to describe parameters.

[irm-codebase]

Most things look great so far! I only found one funky detail in the schema:

Yeah, this is an issue with the upstream dependency. It can't handle "OneOf"/"AnyOf"/"AllOf" schema elements. Another reason perhaps to have a different place to describe parameters.

In that case I'd recommend to add parameters to the math section (as a table, maybe?). To be honest, I am struggling a bit when translating an older model (the Italy one) to test the pathways because of this, since not all cases are listed.