Write a comprehensive how-to guide on preparing an LTS update

The objective of this issue is to produce a single document that every LTS contributor (whether a part of the regular team or not) can use as a checklist for preparing an update. The document should span the process from the initial steps of assessing the state of a package in the work queue, through the publication of the resulting advisory, as well as the various possible alternate resolutions (e.g., all CVEs are triaged as ignored/postponed/not-affected, the package is recommended for EOL, etc.)

Note that care should be taken to survey existing documentation, and that part of the consolidation effort includes migrating documentation from various locations into the canonical document/location. Additionally, where appropriate detailed specialty information should be provided in separate documents and linked from the canonical document.