doc: update link for landing PRs

The onboarding doc links to the "Technical How-to" portion of the Collaborator's Guide for landing PRs. That section is a subsection of the "Landing Pull Requests" portion of that document. By skipping the main section header, important information is skipped, such as the part about not using the merge button and descriptions of the metadata required. Update the link to target the main section and not the subsection.

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc