Create a containerd installation guide and a new `kata-manager` script
for 2.0 that automated the steps outlined in the guide.
Also cleaned up and improved the installation documentation in various
ways, the most significant being:
- Added legacy install link for 1.x installs.
- Official packages section:
- Removed "Contact" column (since it was empty!)
- Reworded "Versions" column to clarify the versions are a minimum
(to reduce maintenance burden).
- Add a column to show which installation methods receive automatic updates.
- Modified order of installation options in table and document to
de-emphasise automatic installation and promote official packages
and snap more.
- Removed sections no longer relevant for 2.0.
Fixes: #738.
Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>
