Improve website interface

[redeboer]

### Tasks
- [x] Reflect `docs/` folder structure on page structure on the website using [`toctree`](https://myst-parser.readthedocs.io/en/stable/syntax/organising_content.html#using-toctree-to-include-other-documents-as-children)
- [x] Add button with [link to GitHub repo](https://sphinx-book-theme.readthedocs.io/en/stable/components/source-files.html#add-a-link-to-your-repository)
- [x] Change download buttons to [links to source code](https://sphinx-book-theme.readthedocs.io/en/stable/components/source-files.html#add-a-button-to-the-page-source)
- [x] Add button to [suggest edits](https://sphinx-book-theme.readthedocs.io/en/stable/components/source-files.html#add-a-button-to-suggest-edits) and [issues](https://sphinx-book-theme.readthedocs.io/en/stable/components/source-files.html#add-a-button-to-open-issues)
- [x] Add "under active development" [banner](https://sphinx-book-theme.readthedocs.io/en/stable/sections/announcements.html#announcement-banners)
- [x] Add [interactivity buttons](https://sphinx-book-theme.readthedocs.io/en/stable/content/launch.html)
- [x] Add [favicon](https://github.com/ComPWA/policy/blob/0d7784c8624cb103e720ba4eafe19446260eee03/docs/conf.py#L63) (see [this one](https://github.com/ComPWA/policy/blob/69aa3b419c300639ac3ee6c4e0d7f451f1576e90/docs/_static/favicon.ico)).
- [x] Add [GlueX logo](https://github.com/user-attachments/assets/5650480b-8bed-4fb2-9f2b-c43b12e16ac3) or ComPWA logo in sidebar
- [x] ~[Optional] Switch to [`sphinx-external-toc`](https://sphinx-external-toc.rtfd.io)~

See here for inspiration.

[redeboer]

This issue is best done before #60, so that we can have an ampform-dpd/ directory next to the existing ampform/ directory

[redeboer]

@shenvitor, in Sphinx documents, you usually organise content with toctree. I would try that first. Later on, we may try to organise with sphinx-external-toc as well.

[redeboer]

Suggested structure:

index.md  # <-- main toctree
branching-fractions.md
LambdaKpi.md  # <-- sub toctree, NEW
LambdaKpi/
    ampform.ipynb
    ampform-dpd.ipynb  # next PR!
    manual.ipynb
etapip.md  # <-- sub toctree, originally reaction.md
etapip/
    ampform.ipynb
    manual.ipynb

[shenvitor]

index.md  # <-- main toctree
branching-fractions.md
lambda-k-pi/
    index.md  # <-- sub toctree, NEW
    ampform.ipynb
    ampform-dpd.ipynb  # next PR!
    manual.ipynb
eta-pi-p/
    index.md  # <-- sub toctree, originally reaction.md
    pgamma-state.ipynb
    ampform.ipynb
    manual.ipynb

[redeboer]

index.md  # <-- main toctree
branching-fractions.md
lambda-k-pi/
    index.md  # <-- sub toctree, NEW
    automated.ipynb
eta-pi-p/
    index.md  # <-- sub toctree, originally reaction.md
    pgamma-state.ipynb
    automated.ipynb
    manual.ipynb

And then in #60, we rename automated to ampform 😁 Because then there are two 'automated' ways of building the amplitude model.

[redeboer]

Btw, you can later on set redirects on your Read the Docs page (gluex-nstar.rtfd.io).

[redeboer]

For the banner, see e.g. here: https://github.com/ComPWA/polarimetry/blob/83d1c3fb0b55ef27504cd1db700b9c09a0f22640/docs/conf.py#L292-L297