6877 navbar guide url

[poikilotherm]

What this PR does / why we need it:
Please see the discussion in #6877 for details (I don't like to repeat that here... 😉 )

Which issue(s) this PR closes:

Closes #6877
Relates to #3637

Special notes for your reviewer:
None.

Suggestions on how to test this:
Deploy. Set the new setting. Check if the "User Guide" link in the navigation bar is pointing to the new location.

Does this PR introduce a user interface change?:
Yes, but @TaniaSchlatter said it's fine.

Is there a release notes update needed for this change?:
Maybe? It really is a very simple and small change.

Additional documentation:
None.

[coveralls]

Coverage remained the same at 19.63% when pulling 5cec042 on poikilotherm:6877-navbar-guide-url into 5ffc7b0 on IQSS:develop.

[pdurbin]

Looks good. The issue has a lot of discussion about this feature.

[mheppler]

@poikilotherm the whole point of that initial question, "What this PR does / why we need it:" in the PR template is so that the developer can provide everyone a brief description of what these changes provide, without requiring them to read an entire conversation in an issue with dozens of comments. 👍

[poikilotherm]

@poikilotherm the whole point of that initial question, "What this PR does / why we need it:" in the PR template is so that the developer can provide everyone a brief description of what these changes provide, without requiring them to read an entire conversation in an issue with dozens of comments. +1

You are absolutely right. Normally I add sophisticated writeups in that section. This time, for this very small change and @pdurbin already done with the review before the PR even existed, I didn't want to re-emphasize... Promise I'll do this for other PRs.