Fix rendering LaTeX in documentation.

[1uc]

Use imgmath_embed = True to work around a path bug for files/notebooks located in subfolders.

Use imgmath_image_format = 'svg' because PNG results in pixalated images.

[1uc]

The locally rendered documentation looks like this:

The math images scale nicely as one zooms and, as expected, are crystal clear and give off good LaTeX vibes.

[1uc]

Closes https://github.com/BlueBrain/nmodl/issues/1095

[codecov[bot]]

Codecov Report

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

Project coverage is 88.38%. Comparing base (9df433b) to head (4f87643).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1167 +/- ## ======================================= Coverage 88.38% 88.38% ======================================= Files 176 176 Lines 12946 12946 ======================================= Hits 11442 11442 Misses 1504 1504 ```

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[bbpbuildbot]

Logfiles from GitLab pipeline #195665 (:white_check_mark:) have been uploaded here!

Status and direct links:

• :white_check_mark: spack_setup
• :white_check_mark: build:intel
• :white_check_mark: build:nvhpc
• :white_check_mark: test:intel
• :white_check_mark: test:nvhpc