Deprecation/versioning documentation

[ananthsub]

How should we communicate Lightning's versioning & deprecation on the docs? The current procedure of mark as deprecated in version N and deprecate in minor version N+2 isn't mentioned anywhere, but I think this would be valuable to communicate that:

• Lightning has a formal procedure through which we go through API deprecations
• Lightning takes backwards compatibility very seriously
• Lightning as a project is perpetually evolving and improving

I think it'd also be useful to publish:

• Schedule for patch versions, and what they contain (bugfixes)
• Schedule for minor versions, and what they contain (e.g. new features, bugfixes, deprecations)
• What would qualify/necessitate a new major version

@edenafek

[justusschock]

cc @edenlightning

[awaelchli]

I opened #9025 to kickstart this.