New maintainer guide (#4472)

* Draft the new maintainer guide

* Maintainer guide - Apply suggestions from code review

Co-Authored-By: Raihaan Shouhell <raihaanhimself@gmail.com>
Co-Authored-By: Tim Jacomb <t.jacomb@kainos.com>
Co-Authored-By: Daniel Beck <1831569+daniel-beck@users.noreply.github.com>

* Apply suggestions from code review

Co-Authored-By: Jeff Thompson <37345299+jeffret-b@users.noreply.github.com>
Co-Authored-By: Francisco Fernández <31063239+fcojfernandez@users.noreply.github.com>

* Maintainer guide - Replace the first role by "contributor"

* Maintainer guide - Add more details

* Maintainer guide - address some feedback

* Apply suggestions from code review by @daniel-beck

Co-Authored-By: Daniel Beck <1831569+daniel-beck@users.noreply.github.com>

* Update .github/PULL_REQUEST_TEMPLATE.md

Co-Authored-By: Daniel Beck <1831569+daniel-beck@users.noreply.github.com>

* Maintainer guide - more updates

* Maintainer Guide: Add info about the Security Releases and merges + add an explicit Jenkins LTS section

Addresses comments from @daniel-beck and @olivergondza

* Maintainer guide: reference the advisories

* Apply suggestions from code review

Co-Authored-By: Mark Waite <mark.earl.waite@gmail.com>
Co-Authored-By: Raihaan Shouhell <raihaanhimself@gmail.com>
Co-Authored-By: Daniel Beck <1831569+daniel-beck@users.noreply.github.com>

* Apply suggestions from code review

Co-Authored-By: Mark Waite <mark.earl.waite@gmail.com>

* Fix typos

* Expand code review goals: documentation and code styles

* Maintainer guide: Document the squash-merge policy

* Apply suggestions from code review

Co-Authored-By: Mark Waite <mark.earl.waite@gmail.com>

Co-authored-by: Raihaan Shouhell <raihaanhimself@gmail.com>
Co-authored-by: Tim Jacomb <t.jacomb@kainos.com>
Co-authored-by: Daniel Beck <1831569+daniel-beck@users.noreply.github.com>
Co-authored-by: Jeff Thompson <37345299+jeffret-b@users.noreply.github.com>
Co-authored-by: Francisco Fernández <31063239+fcojfernandez@users.noreply.github.com>
Co-authored-by: Mark Waite <mark.earl.waite@gmail.com>