[MNT] Remove auto generated API page from toctree

[MatthewMiddlehurst]

Fixes #614

This removes all the API pages from the sidebar apart from the navigation pages using sphinx-remove-toctrees. This was an option suggested in pradyunsg/furo#674, where others have also faced large build times with auto-generated API pages.

As a consequence, the auto-generated API pages will no longer appear in the sidebar or show the current page when on an auto-generated API page. The pages still exist and can be searched using the search bar.

Also removes some old functions and a css file used in the "estimator overview" page prior to its removal.

[MatthewMiddlehurst]

New build times:
768s https://readthedocs.org/projects/aeon-toolkit/builds/21462472/
790s https://readthedocs.org/projects/aeon-toolkit/builds/21462636/

This is well below the last two failures due to build time:
1915s https://readthedocs.org/projects/aeon-toolkit/builds/21437960/
1912s https://readthedocs.org/projects/aeon-toolkit/builds/21438686/

and still below some of the non-timeout builds done recently:
1099s https://readthedocs.org/projects/aeon-toolkit/builds/21451131/
1159s https://readthedocs.org/projects/aeon-toolkit/builds/21451337/
1155s https://readthedocs.org/projects/aeon-toolkit/builds/21461167/

I honestly have no idea why the builds were dropping out still, it seems like a major increase in build time with no readily obvious reason. It could just be unrelated to us, as it happened to multiple PR and main branch builds with no pattern.

Either way, this change makes our docs builds faster and more sustainable as we add to it IMO.