docs: fix intro notebook, and submodule and subpackage index

[Saransh-cpp]

Issues

• The intro notebook is currently broken because of missing extra \s when writing latex.
• There are no differences between H2 and H3 headings which makes the index in the documentation behave weirdly.

Changes done

1. All the submodule headings are now H3 and the headings saying "Submodule" are H2.
2. Vector's direct submodules have their independent rst files and pages now (The documentation for the submodules currently renders directly in the index page).
3. Added information about vector on the landing page.
4. Fixed the intro notebook.

Screenshots

Sidebar index

Current

New

vector index

Current

New

vector package index

Current

New

Direct submodule documentation

Current

New

Contents bar

Current

New

[henryiii]

I'd recommend running pre-commit run -a locally to see what it's complaining about. See https://scikit-hep.org/developer/style.

[Saransh-cpp]

Thank you! Installed pre-commit locally!

[henryiii]

Thanks!