docs: add guides on PR benchmark and cross-release benchmark

[JuroOravec]

This PR documents integration with GitHub for these use cases:

• Performance report on pull requests.
• Benchmarking the package across releases.

I'm not sure how to run Sphinx locally, so I didn't check if the docs build, and if the images are linked correctly. But I at least used this website Documatt Snippets to ensure that the rst syntax is correct (I use markdown).

Closes #796

[mattip]

I did this to build the docs

pip install -e .      # to install asv
pip install asv[doc]  # to get the requirements
sphinx-build -W docs/source/ html

I don't understand why the CI did not run here.

[mattip]

I get a warning, why is the image unreadable is missing?

asv/docs/source/guide_pull_request.rst:17: \
    WARNING: image file not readable: _static/asv-report.png [image.not_readable]

[mattip]

LGTM (once the missing image is found). It might be nice to try to resolve some of the redundant information in the new docs and the existing user guide

[JuroOravec]

Note to self - I'll have to update the docs / code - getting a github bot to comment on PRs was actually trickier because of permissions issues.