Clarify best practices with ref target labels in docs

On python/cpython#94636 , specifically https://github.com/python/cpython/pull/94636#discussion_r934008442 , @ezio-melotti , @erlend-aasland and I discussed that that it might be helpful to clarify in the devguide some additional guidance related to reference target labels in the Cross-linking markup section.

Specifically, it could mention that whenever possible, the existing ref target should be left place (either instead of or addition to adding a new one) when sections are changed or moved, since it ensures any inbound internal or Intersphinx references don't break or need to be changed, as well as any external links that anchor (provided that it wasn't moved to a different page without a redirection).

Also, it could provide a guideline and examples on how ref labels should be "namespaced" to avoid conflicts, i.e. by the module name for library docs, or the page name elsewhere, documenting existing (if not always consistent) convention.

If we agree this would be helpful, I can submit a PR on this once #916 is merged.