Improve documentation

[oshoma]

• fix typos
• add topic headings to navigation sidebar
• remove sidebar navigation sections with only 1 child node
• hoist top-level toctree statements up to docs/index.rst
• fix install instructions for working on docs
• remove words to make text more up-to-date concise
• remove stale Slackbot tutorial info

...and there's still more to do. :)

Description

Here's a video showing the main changes.

To build the docs yourself and see the changes:

cd docs
python -m venv .venv
source .venv/bin/activate
pip install --upgrade pip
pip install -r requirements.txt
make html
open _build/html/index.html

...and then compare what you see to https://sherpa-ai.readthedocs.io/en/latest/index.html.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Maintenance
• New release

Checklists

To speed up the review process, please follow these checklists:

Development

• The Pull Request is small and focused on one topic
• Lint rules pass locally (make format && make lint)
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development (make test)
• The changes generate no new warnings (or explain any new warnings and why they're ok)
• Commit messages are detailed
• Changed code is self-explanatory and/or I added comments
• I updated the documentation (docstrings, /docs)
  See the testing guidelines for help on tests, especially those involving web services.

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached.
• I have performed a self-review of my code
• Issue from task tracker has a link to this pull request

💔 Thank you for submitting a pull request!