Kiwix's policy on supporting various platforms

During a live discussion with @kelson42 we recognized the need to have a well-defined policy on our commitment of supporting various platforms.

Regarding Linux we reached the consensus that 2 most recent LTS releases of Ubuntu should be supported. However that is not as accurate as it should be. Consider the date when a new LTS release of the OS is announced. With our current development and release management process there is some chance that certain `kiwix` libraries will fail to build or work on that new version of the OS and will take some time to be fixed.

This ticket is intended to formalize the policy and have it properly published so that our users know what they can expect in that regard.

The issue names no target file, test, or documentation entry point. Start by reviewing the current Linux and Ubuntu support statements alongside the release-management process; done means the agreed platform-support policy and transition timing are published clearly for users.

Written by the indexing model from the issue text.