Update link to Issue board focus mode

Why is this change being made?

Follow-up from documentation changes in gitlab-org/gitlab!28597 (merged).

This updates the link to the Issue board focus mode feature. Because it's being moved to another tier, the section's heading (and the link) has changed.

Related issues

gitlab-org/gitlab#212331 (closed)

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what