Add `insert_anchor_links` back to `config.toml`

insert_anchor_links was a valid config.toml setting before 0.0.6 but was changed to a section option.

It was mentioned in the documentation for config.toml until 2021 (#1477).

In 2021, Keats said that insert_anchor_links could probably be a config option in config.toml”.

In the same thread, several other users requested this feature. Artoria2e5 said:

These sort of settings become annoying to copy-paste when a site ends
up with many sections.

ctron said:

But even more annoying is that it seems I can’t add
insert_anchor_links on the top section, but need to place it in every
section.

I think having this as a default configuration for the site makes
total sense.

I wrote a draft implementation of this feature #2767 because it looked pretty easy and luckily the diff is pretty small (+57 -10 including docs and tests).