Main goal: consolidate the information about image
building under the same documentation page, and move
plugin-specific details inside plugin pages.
No plugin-specific information should live outside
those pages.
More details:
- move the detailed documentation about sahara-image-pack
from the contributor guide to the new dedicated page
in the user manual;
- remove the vanilla and cdh pages which describes building
images with sahara-image-create, and move the common
information to new sahara-image-create page
in the user manual;
- add the matrix of supported plugin versions and
supported building technology for each plugin inside
the respective <plugin>-plugin.rst;
- add the redirects for the removed pages (only for master
and rocky, where this change should be backported).
- remove few details not really needed (e.g. how to convert
to VMDK images, location of cloud-init packages, etc,
which do not really belong here).
Change-Id: I8398a7ad625276d8f11d743688ba71902a7e1adc
4a44010f31