Document the support policy for optional packages

[seisman]

Preview: https://pygmt-dev--3616.org.readthedocs.build/en/3616/minversions.html

Closes #3456.

[seisman]

ensuring at least two latest minor versions remain supported.

I've added this sentence to the GMT support policy.

[weiji14]

Also debating on whether to add something based on this note from SPEC 0:

Core packages may or may not decide to provide bug fix releases during the full 2 year period after release. Therefore, projects may occasionally want to drop support for core package dependencies earlier than recommended by this SPEC. For instance, if a newer minimum version of a core package is needed by a project due to a critical bug fix, which is not backported to older versions.

E.g. we have pinned to xarray>=2023.04 at #3460 earlier than expected.

[seisman]

Also debating on whether to add something based on this note from SPEC 0:

Core packages may or may not decide to provide bug fix releases during the full 2 year period after release. Therefore, projects may occasionally want to drop support for core package dependencies earlier than recommended by this SPEC. For instance, if a newer minimum version of a core package is needed by a project due to a critical bug fix, which is not backported to older versions.

E.g. we have pinned to xarray>=2023.04 at #3460 earlier than expected.

Maybe add a note like:

The policy is not strictly enforced and occasional breaches of the policy are allowed after discussions within the PyGMT team.