Add toc in source page

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (223b230) 94.67% compared to head (2f3c3ee) 94.81%.

Files	Patch %	Lines
src/mkapi/pages.py	95.94%	3 Missing :warning:
src/mkapi/plugins.py	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #85 +/- ## ========================================== + Coverage 94.67% 94.81% +0.14% ========================================== Files 14 14 Lines 2196 2217 +21 ========================================== + Hits 2079 2102 +23 + Misses 117 115 -2 ```

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