This patch adds single page that contains
list of all plugins with their description.
In the future next things will be addressed:
- Split this page into few pages, each page should
contain only specific type of plugins
- Improve descriptions of plugins that we have,
especially description of context classes
- Add description to base plugin classes that expalins
how everything works.
So it won't be hardcoded in plugins.rst and it will
be possilbe to use it in different places
Change-Id: I87ec6f8497392306578d91263c98c4c035acc521
206eba1809