Add last modified date to all pages.

[#69382658]

Signed-off-by: Nikhil Gajwani <ngajwani@pivotallabs.com>
Refactors file-update-time feature. Places cache-prettifying logic in GitModCache.

Signed-off-by: Nikhil Gajwani <ngajwani@pivotallabs.com>
Ignore the file_modification_dates cache file.

Signed-off-by: Nikhil Gajwani <ngajwani@pivotallabs.com>
Handle modified_times for pages not in the cache, and formatting.

Signed-off-by: Nikhil Gajwani <ngajwani@pivotallabs.com>
Update the README to include the  helper.

Signed-off-by: Nikhil Gajwani <ngajwani@pivotallabs.com>