Fix some broken anchor links in the project documentation

What does this merge request do and why?

I noticed recently that a few of our links were broken.

We do have a link checker, but it can't check anchors on other different files until https://github.com/tcort/markdown-link-check/issues/212 is resolved. So we have partial coverage of broken links, but the rest need to be checked locally.

This MR fixes broken anchor links I've found.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.