doc: genrest: Use Kconfiglib 2 and improve output

The new version of the reference page for a symbol lists the following
essential and generally useful information at the top of the page:

 - Prompt

 - Type

 - Help text

 - Direct dependencies (from 'depends on', and inherited from if's and
   menus)

 - Defaults

 - Other symbols selecting or implying the symbol

The current symbol value and visibility are no longer displayed. They
don't make much sense in a reference document, as they depend on user
values from configuration files.

The bottom of the reference page also includes the symbol represented in
Kconfig format (including propagated dependencies).

All references to defined symbols appearing in expressions are turned
into links, which can be clicked on to jump to the reference page for
that symbol.

The following (slightly outdated) screenshot gives an idea of the
format:
https://www.dropbox.com/s/a34tlk2ncyus8po/promptandtype.png?dl=0



This commit also removes Kconfiglib 1, since it is no longer used.

Fixes #5622
Fixes #6821

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>