* link_checking: prevent rate-limiting
Fix for https://github.com/getzola/zola/issues/1056.
- assign all links for a domain to the same thread
- reduce number of threads from 32 to 8
- add sleep between HTTP calls
* Add get_link_domain(), use for loops
* Do not sleep after last link for domain
* Avoid quadratic complexity
* remove prints
This updates rendered markdown structures in order to keep track
of all internal links, not anymore limiting to only those targeting
an explicit anchor fragment.
The goal of this rework is to allow building other features, such
as backlinks, on top of the existing collection of internal links.
