doc: improve collaborator guide, doc fast-tracking process

doc: reorganize collaborator guide

• Add sections about first time contributions, code reviews and seeking consensus, waiting for approvals, testing and CI
• Move paragraphs to more suitable sections
• Update table of contents

doc: document the fast-tracking process

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc