Add date of build / commit hash as variable

[Tom-van-Woudenberg]

This variable could be used on any page / footer / etc, to inform the reader about the version / date they're reading

[Tom-van-Woudenberg]

Can be done by adding

sphinx:
  config:
    html_last_updated_fmt: '%B %-d %Y'
  extra_extensions:
    - sphinx.ext.extlinks