Review guidance for the Debian and RPM installation instructions to use variables

[yetanothertw]

While working on #2234, we've identified that improvements around version variable usage can be made to the following installation instruction pages:

• Install Elasticsearch with a Debian package
• Install Elasticsearch with RPM

This applies specifically to the sections that describe installing from the APT and RPM repositories respectively. These instructions could be improved by adding information on how to install specific versions of Elasticsearch, in an separate tab for each applicable step. This would be similar to the approach used in PR#2426.

Additional links: Version Variables

[yetanothertw]

pinging @eedugon for when you're back from PTO