HA restructure/re-writes for brevity & simplicity

[cwarnermm]

Restructured & rewrote the High Availability product docs to:

• reduce verbosity while maintaining clarity
• focuse on essential configuration while separating advanced topics into links or references for users who need them.
• align better with the workflow admins would follow when setting up a high-availability (HA) environment for the first time.
• organize the order of info as a bottom-up approach, starting with foundational infrastructure (time, database, file storage) before progressing to application-layer configuration and advanced settings like job servers, plugins, and CLI usage. It ensures admins address critical dependencies first, avoiding potential misconfigurations later on.

Review proposed updates & compare against published docs.

Outstanding

• Fix redirects for changed H2s & H3s
• Fix broken links due to changed H2s & H3s

[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA 8524e0f