📚 Various textual/flow improvements

[ltalirz]

• developers: link to "customise" section instead of repeating content
• maintainers: split off release docs into separate file

Various textual/flow improvements

[chrisjsewell]

FYI for reviewing these docs are being built for PRs, but as mentioned annoyingly they are not currently being "pushed" back to the PR checks: https://quantum-mobile--140.org.readthedocs.build/en/140/