Include link to pantsbuild.org repo and commit in footer

[huonw]

This adjusts the "copyright" section of the website footer to include:

• a link to this repository, replacing the "built with docusaurus" text, since that can be discovered by clicking through to the repo (it be fine to continue to include that if required).
• a reference to which commit the deployed website is showing, so we can be sure we're looking at what we expect (it would be fine to move this to a HTML comment instead, so it's just available for reference in the rare case it's required).

It also removes the year, since having that auto-update is technically incorrect.

Example:

[thejcannon]

• I just left the Docusaurus tidbit in there because the tech stack is great to work with overall, and the support I got on GitHub and discord was good. Not required at all though.
• the code commit is a good idea, but it feels weird to put it in the footer. I don't think I feel strongly either way though so I'll ping @benjyw and let him decide

[benjyw]

I know using the current year is common practice, but it's not actually correct. The copyright year should reflect the year of first publication, not the current year (since that conveys no information relevant to the content for which common law copyright is being claimed). So to be meaningful year should be the year of first publication, or the last time the file was substantially modified (if that can be measured).

An easier possibility is to omit the year entirely...

[huonw]

Removed!