Specify the developer policy around links to external resources

This specifies the developer policy on adding links in source & test
files and commit messages, related to discussion at:
https://discourse.llvm.org/t/code-review-reminder-about-links-in-code-commit-messages/71847

The intent is to discourage adding links to resources that are not
available to the community as a whole (dead links, links to internal
documentation, links to internal bug trackers, etc) from source and
test files, while still allowing such links to appear in other contexts
as needed. It suggests to instead add sufficient context in the
surrounding comments to make such links unnecessary.

It also clarifies that these links can appear in commit messages as
metadata (similar to how we already have Differential Revision and
Fixes metadata with links).

Differential Revision: https://reviews.llvm.org/D155081