genrest: List all symbols on main index page and refactor

Instead of having index.rst just link to other index pages in --modules
mode, list all symbols on it, and have links to the module index pages
at the top.

Get rid of index-all.rst and index-main.rst (and all related options).
index-all.rst is no longer needed, and index-main.rst (symbols outside
--modules) might not be that useful to people reading the documentation
(and could be added back if needed).

Also refactor and streamline the code a bunch. For the main index page,
the only difference between modules and non-modules mode is now whether
there's links to other index pages at the top.

Suggested by David B. Kinder.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>