Modules API docs: Sections and links (redis#8442)

* Modules API docs: Link API function names to their definitions

Occurrences of API functions are linked to their definition.

A function index with links to all functions is added on the bottom
of the page.

Comment blocks in module.c starting with a markdown h2 heading are
used as sections. A table of contents is generated from these
headings.

The functions names are changed from h2 to h3, since they are now
rendered as sub-headings within each section.

Existing sections in module.c are used with some minor changes.
Some documentation text is added or sligtly modified.

The markdown renderer will add IDs which may clash with our
generated IDs. By prefixing section IDs with "section-" we make
them different.

Replace double dashes with a unicode long ndash