Clarify C family SDK installation.

The Python install instructions rely upon the Couchbase release deb.
This does not support installation on recent Debian, nor latest LTS Ubuntu (18.04/Bionic).

The PHP install on GitHub refers directly to the C Release notes, which contain install instructions.

We have a contributed PR - https://github.com/couchbase/docs-sdk-python/pull/3/files - on working around the couchbase-release deb shortcomings. The instructions look good, and I've tested it on a clean 18.04 vagrant.

Looks like we need to:

1. Clarify the support position of all SDK versions on Deb9 & Ubuntu 18.04
2. Consider removing the LCB install instructions from Python _et al._ and referring to LCB (only one source to keep up-to-date).
3. Clarify how much of the install instructions we need in both Start Using... and in Release Notes – possibly pulling in a common page-partial.