[x] Based your PR on the latest version of the correct branch (master or 4.x)
[x] Checked your writing carefully for correct English spelling, grammar, etc
[x] Described your changes and the motivation for them below
Description of Changes
Currently, the warning banner for old (and new) versions embeds a link title and URL that hardcode the version number of the latest stable docs. With Sphinx-Multiversion that was fine, since all branches were rebuilt in sync so even the old banners would always be up to date, but with the new theme introduced in #367 , this will no longer be the case and unmaintained branches will be static, and not migrated to the new theme. Therefore, we need to update the old banner to be "evergreen" and point to the /current/ version rather than a hardcoded version value.
I'll also fix any linkcheck issues that may come up.
Pull Request
Pull Request Checklist
Description of Changes
Currently, the warning banner for old (and new) versions embeds a link title and URL that hardcode the version number of the latest stable docs. With Sphinx-Multiversion that was fine, since all branches were rebuilt in sync so even the old banners would always be up to date, but with the new theme introduced in #367 , this will no longer be the case and unmaintained branches will be static, and not migrated to the new theme. Therefore, we need to update the old banner to be "evergreen" and point to the
/current/version rather than a hardcoded version value.I'll also fix any linkcheck issues that may come up.
Required for #367