✅ SEP 19: Add a sitewide count of backlinks

Status: Finished 

Date: 2023-04-08 
Commit: e521f96aa448b83f61e25b9f986e17456934ec15
> Implemented in the `build_backlinks()` function with a simple deduplicated
> counter on total backlinks inserted. Stored in `site['backlink_count']`.
> Already put to use on the /stats page.

This should be trivial to implement. Simply tally up the total number or interlinks between documents and store this in the site metadata so that it can be accessed when generating stats pages and also for debugging purposes.

To be implemented in the build_backlinks() function.