Fix internal links in READMEs

[bjee19]

Certain internal links on our READMEs do not work correctly and link to a 404 not found page. They seem to be internal links that are in a Note.

Example failure: https://github.com/nginxinc/nginx-gateway-fabric/blob/main/docs/guides/upgrade-apps-without-downtime.md?plain=1#L8

Acceptance Criteria:

• Fix all links.
• Communicate with Github support to see why our original method is not working and perhaps open a bug report with them.

[bjee19]

Looks like there is already a discussion open on the GIthub community where others are having similar issues: https://github.com/orgs/community/discussions/69994