Fix relative links

[reakaleek]

Context

In #436 (comment) we discovered an unexpected behaviour of relative links. (unexpected but correct behaviour)

TL;DR

If you are on a page with relative links, your link resolves to something different depending if the current path has a trailing slash or not.

Reproduce

1. Go to https://docs-v3-preview.elastic.dev/elastic/docs-builder/pull/436
2. Click on the first link "Contribute to Elastic documentation"
3. It leads to a non existing link.

1. Go to https://docs-v3-preview.elastic.dev/elastic/docs-builder/pull/436/ (trailing slash)
2. Click on the first link "Contribute to Elastic documentation"
3. It works

Changes

• This transforms every relative path to a root-relative path (starting with /) to avoid the problem of relative links.
• Also, it's sets /docs as url path prefix in tests as this resembles production.

How to test

1. Go to https://docs-v3-preview.elastic.dev/elastic/docs-builder/pull/442
2. Click on the first link "Contribute to Elastic documentation"