docs: Fix broken link to user guide

[jannfis]

Fixes broken link to user guide on the start page. As reported in #4468 (comment)

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• Optional. My organization is added to USERS.md.
• I've signed the CLA and my build is green (troubleshooting builds).

[codecov-commenter]

Codecov Report

Merging #4477 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4477   +/-   ##
=======================================
  Coverage   41.17%   41.17%           
=======================================
  Files         122      122           
  Lines       16588    16588           
=======================================
  Hits         6830     6830           
  Misses       8776     8776           
  Partials      982      982           

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