fix: static content for routePrefix with subpath (#165)

[miraries]

This fixes #165, where having a subpath in routePrefix breaks static content loading due to changes in #164.

Now the static content urls are relative only with a trailing slash, otherwise they're absolute - meaning they work the same as before the regression.

The tests for this were already developed but with wrong assertions, to be able to test static content is loaded I assume we'd need to run playwright with different prefix variations, which would require a refactor as all playwright tests are run with /documentation as the prefix.
Please let me know if you have any suggestions regarding the tests.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm

[KharchenkoDmitriy]

Experienced related issue with swagger-ui in module
When register swagger ui on top level with routePrefix: "/docs" it works
When register it inside a module with routePrefix: "/docs", it cannot serve static

Reading this PR guide me to the solution - routePrefix: "docs" which solve static serving issue
Not sure if it is expected behavior but it would be nice to mention it in Docs/Readme