fix: relative links in documentation

Use consistently relative paths when linking to content from other files such that links are correctly resolved in both plain markdown and vitepress.

Pull Request Checklist

[ ] 1. I have rebased the latest version of the main branch into my feature branch and all conflicts have been resolved.
[ ] 2. I have added information about the change/addition to functionality to the CHANGELOG.md file under the [Unreleased] heading.
[ ] 3. I have documented my code using JDocs tags.
[ ] 4. I have removed unnecessary commented out code, imports and System.out.println statements.
[ ] 5. I have written JUnit tests for any new methods/classes and ensured that they pass.
[ ] 6. I have created API tests for any new functionality exposed to the API.
[ ] 7. If changes/additions are made to the ors-config.json file, I have added these to the ors config documentation along with a short description of what it is for, and documented this in the Pull Request (below).
[ ] 8. I have built graphs with my code of the Heidelberg.osm.gz file and run the api-tests with all test passing
[ ] 9. I have referenced the Issue Number in the Pull Request (if the changes were from an issue).
[ ] 10. For new features or changes involving building of graphs, I have tested on a larger dataset (at least Germany), and the graphs build without problems (i.e. no out-of-memory errors).
[ ] 11. For new features or changes involving the graphbuilding process (i.e. changing encoders, updating the importer etc.), I have generated longer distance routes for the affected profiles with different options (avoid features, max weight etc.) and compared these with the routes of the same parameters and start/end points generated from the current live ORS. If there are differences then the reasoning for these MUST be documented in the pull request.
[ ] 12. I have written in the Pull Request information about the changes made including their intended usage and why the change was needed.
[ ] 13. For changes touching the API documentation, I have tested that the API playground renders correctly.

Fixes #1789.

Information about the changes

Key functionality added: switch to valid relative links.
Reason for change: enable correct links when navigating through markdown sources.

GIScience / openrouteservice

fix: relative links in documentation #1791

Pull Request Checklist

Information about the changes

Quality Gate passed