[doc] update release-process.md and backports section of CONTRIBUTING

- Mention which directories contain the respective unsigned tarballs
- Clarify that bitcoin.conf might not need to be updated
- Specify where to put historical release notes if there is already
  something in release-notes.md
- Clarify what exactly is the problem with running guix-codesign more
  than once
- Correct number: 6 codesigned attestations are needed before uploading
  binaries
- Remove scp command which is outdated
- Remove server path which is outdated
- Specify that translations update should happen before branch-off, not
  before each release candidate
- Mention that you should notify lists when RCs are available
- Put "Archive the release notes" as a separate step, since creating the
  github release has a dependency on it.
- Put bitcoincore.org website updates as a separate step, since
  updating packaging repos may have a dependency on it.
- Update "bitcoin-dev mailing list" to "bitcoin-dev group"
- Document that maintainers should create PRs to collect backports
- Remove section about not uploading `*-debug` files, reader should
  upload all build artifacts.
- Torrent is created automatically, so delete instructions.
- Mention that server also generates ots file automatically.

CONTRIBUTING.md

@@ -417,11 +417,8 @@ Backporting
 
 Security and bug fixes can be backported from `master` to release
 branches.
-If the backport is non-trivial, it may be appropriate to open an
-additional PR to backport the change, but only after the original PR
-has been merged.
-Otherwise, backports will be done in batches and
-the maintainers will use the proper `Needs backport (...)` labels
+Maintainers will do backports in batches and
+use the proper `Needs backport (...)` labels
 when needed (the original author does not need to worry about it).
 
 A backport should contain the following metadata in the commit body: