Add documentation and devtools for compat module updates

In the scope of #7000, a new module "compat" was introduced. The goal of this new module is to provide a compatibility configuration layer to make Tarantool updates less painful for users. In a nutshell, when a breaking change is about to be introduced, we should add a new compat option for it and set it to 'old', which means the old behavior (i.e. no breaking change actually happens). When the major version is bumped, we should:

• update all 'old' compat options to 'new', but allow to reset them to 'old' manually;
• update all 'new' compat options to 'deprecated' and disable/drop the corresponding code.

However, the procedure wasn't properly documented anywhere. It'd be nice to have the documentation right in the repository that would outline the following points:

• What to do on minor release?
• What to do on major release?
• How to add a breaking change?

Ideally, we should also have some scripts that would help automate the compat module update on release.

Start by reading issue #7000 and locating the compat module and its release-related code in the repository. Document the minor-release, major-release, and breaking-change procedures, then determine where release scripts belong; done means the procedure is documented and any agreed automation is usable.