Focus search box when pressing slash

[hugovk]

Fixes https://github.com/python/python-docs-theme/issues/130.

Many websites focus the search box when the forward slash / key is pressed.

For example:

• Furo theme https://pradyunsg.me/furo/quickstart/
• PyPI https://pypi.org/project/python-docs-theme/
• GitHub https://github.com/python/python-docs-theme/issues/130
• YouTube https://www.youtube.com/watch?v=nMekFX2CDVk
• Discord https://discuss.python.org/

Let's do the same.

Demo

https://python-docs-theme-previews--131.org.readthedocs.build/en/131/

[hugovk]

Re-opening as https://github.com/python/python-docs-theme/pull/135 didn't work out.

[hugovk]

Or the re-open button doesn't work after rebasing/force pushing, so here's a new PR: #153.