Document the new process for package deprecations

[criamico]

Related to https://docs.google.com/document/d/1daDzfoxlsPM-s9sMy0kizUcB2Ly91mUXORqw_uYy1Ys/edit?tab=t.0#heading=h.fza257wdtm8f

We need to add some internal docs for developers in https://github.com/elastic/elastic-package/tree/main/docs/howto, to cover the new process that allows to deprecate packages or part of them.

See the following sections of the RFC:

• https://docs.google.com/document/d/1daDzfoxlsPM-s9sMy0kizUcB2Ly91mUXORqw_uYy1Ys/edit?tab=t.0#heading=h.fza257wdtm8f
• https://docs.google.com/document/d/1daDzfoxlsPM-s9sMy0kizUcB2Ly91mUXORqw_uYy1Ys/edit?tab=t.0#heading=h.fnf81q2sgn3f