[site,doc] Consider limiting the size of the sidebar ToC for longer pages

### Description

From https://github.com/lowRISC/opentitan/pull/18411

Some pages have waaay too many headings (e.g. [this](https://opentitan.org/book/hw/ip/otbn/doc/isa.html)), so we need some way to filter the list to a manageable length. Some filtering is done currently (eg. for register maps, the subsections are removed).

Start by inspecting the site documentation sidebar ToC and the existing filtering for register-map subsections. Compare the current list against the long ISA page linked in the issue, then define a manageable behavior for pages with many headings while preserving the existing register-map filtering.

Written by the indexing model from the issue text.