Add support for relative links starting with ./ or / to RelativeLinkFilter

## What does this MR do?

- Allow explicit relative links in Markdown (starting with `./`).
- Allow absolute links (relative to the branch's root directory) in Markdown (starting with `/`).

## Why was this MR needed?

Those link types were not supported before.

## What are the relevant issue numbers?

fixes #19028

See merge request !5586