docs: fix link to nix usage docs

[robjtede]

Changes

Docs. Link fix.

Context

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[CLAassistant]

All committers have signed the CLA.

[rarkins]

Links need to work when hosted on docs.renovatebot.com too

[viceice]

please update PR description why this link needs to be changed.

you shouldn't use the source docs from the repo because we generate the real docs as build steps.

[robjtede]

I clearly was wrong about the relative link setup.