Connect the bookmark editor dialog to the user manual

[DelazJ]

since a dedicated section already exists.

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include Fixes #11111 at the bottom of the commit message
• I have read the QGIS Coding Standards and this PR complies with them
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit
• I have evaluated whether it is appropriate for this PR to be backported, backport requests are left as label or comment

[nirvn]

@DelajZ , are you planning to update the section prior to 3.10 release? Significant changes & improvements there.