Use relative URLs where possible

[honzajavorek]

When reading the https://github.com/apify/apify-docs/pull/1117/files diff I noticed, once again, an absolute link to another page within the docs. I've seen this elsewhere throughout the docs. I think it's a remnant of some past re-orgs of the files, where files which were not part of a single repository now are. It would be nice if these links were identified and changed to local URLs:

• Practically, this is an issue only during development, with links heading to the production website. The change of domain name in the address bar may not be easily noticeable and can cause issues like "I'm refreshing this page for 10th time and the content ain't changing according to my edits?!"
• Theoretically, in case we have some Docusaurus plugins or tooling in the future, it would be vital that it has a correct graph of what is interconnected.

This is very low priority, but in my Academy stints there are sometimes periods of time when I wait for reviews, so I file this issue as a little task idea. Should be an easy search & replace.