docs/develop: introduce intermediate index for each section

The table of content of the top level will include everything that's up
to 2 levels down. Currently this results in a lot of pages and no

docs/develop: introduce intermediate index for each section

The table of content of the top level will include everything that's up
to 2 levels down. Currently this results in a lot of pages and no clear
organization.

Extra index pages allow for more intermediate levels, resulting in less
things being listed in the main page. This way the organization is a bit
clearer, even if this is still not great due to many missing pages and
sections in the documentation currently.

Change-Id: I5bbdd96b3c2ff3334b9ad1b86fd9e7fb00fd63a6
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6008
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

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