Consider enabling `collapse_navigation` by default

[Cadair]

Describe the performance issue

We default to collapse_navigation=False which means our navigation sidebar looks like this:

where the whole tree is navigable from all pages.

if set to True it looks like this:

Building the sunpy documentation with collapse_navigation=True takes 145.82 s.

Details

$ tox -e build_docs -- -D html_theme_options.collapse_navigation=True -aE -j 2
[...]
build_docs: commands[2] /home/stuart/Git/sunpy/docs> python -c 'import pathlib; print("Documentation available under file://{0}".format(pathlib.Path(r"/home/stuart/Git/sunpy") / "docs" / "_build" / "index.html"))'
Documentation available under file:///home/stuart/Git/sunpy/docs/_build/index.html
  build_docs: OK (156.30=setup[9.88]+cmd[0.29,0.29,145.82,0.03] seconds)
  congratulations :) (156.35 seconds)

and building the sunpy documentation with collapse_navigation=False takes 372.00 s.

Details

$ tox -e build_docs -- -D html_theme_options.collapse_navigation=False -aE -j 2
[...]
build_docs: commands[2] /home/stuart/Git/sunpy/docs> python -c 'import pathlib; print("Documentation available under file://{0}".format(pathlib.Path(r"/home/stuart/Git/sunpy") / "docs" / "_build" / "index.html"))'
Documentation available under file:///home/stuart/Git/sunpy/docs/_build/index.html
  build_docs: OK (381.91=setup[9.35]+cmd[0.27,0.27,372.00,0.03] seconds)
  congratulations :) (381.95 seconds)
**

Proposed fix

We could set this as the default in the theme, as astropy have done. Or we could conditionally set it for just sunpy, or just tox builds of the sunpy docs (but not RTD) for example.

[ayshih]

I find the parameter name is a bit misleading: it's not that the navigation is merely collapsed when collapse_navigation=True, but rather that it is no longer possible to expand the navigation for other items (or even see which items have subitems). (Plus, the expanding/collapsing button seems a bit silly when the only items you can expand/collapse are for the hierarchy you are currently in.) I like being able to expand all of the navigation. Do I like it enough to be worth taking over twice as long to render the docs? I don't know.

[Cadair]

I like being able to expand all of the navigation. Do I like it enough to be worth taking over twice as long to render the docs? I don't know.

yep, this pretty much sums it up lol.