Add MySQL docs

[dougharris]

Please fill out as much of this as you can. The platform name and base URL are the minimum required.

Platform name (e.g. "Django", "Ruby"):

Example URL https://dev.mysql.com/doc/refman/8.0/en/delete.html

Base URL controls which URLs the extension will be active for, e.g. https://docs.djangoproject.com/*: https://dev.mysql.com/doc

Documentation pattern a regular expressionthat identifies the version indicator in the URL path, e.g. for django, it's docs.djangoproject.com/[^/]*/(?<version>[0-9]+\.[0-9]+|dev)/

Version selector on page the CSS selector for the link or select list to change versions, e.g. for django, it's ul#doc-versions a: