doc: west: add missing parts for zephyr v1.14

- add glossary terms for important concepts we have to explain often,
  like "west installation"

- add autodoc directives for pulling in west API docs

- add missing documentation for built-in features like west's
  configuration, extension commands, etc.

- add missing documentation for "west sign" extension

- describe the manifest in a self-contained way rather than linking to
  the relevant pykwalify schema, also adding a missing reference to
  "west manifest" in the miscellaneous multi-repo commands list

- move various details regarding history and motivation to why.rst
  among other changes to repo-tool.rst, leaving it closer to a "tell
  me what I really need to know" style overview

- update planned features

Fixes: #14992
Signed-off-by: Marti Bolivar <marti@foundries.io>