Update dashboards to homepage in user settings

Background

During the MR for the Dashboards working group, some inconsistencies in naming of the homepage were discovered. Namely the term of Dashboard had been given to the homepage of GitLab.

Pajamas MR from the dashboard working group:

• feat(Dashboard): Init dashboard definitions (gitlab-org/gitlab-services/design.gitlab.com!3260 - merged)

The new definitions

• Dashboards have a specific definition with panels and certain behaviors
• The home page is a landing page that is accessed through the Avatar
• Any page can be landing page if it comes form a navigation item

Proposal

Reference MRs

Recently are two MRs to change to Start page to Dashboards and this is changing it one more time to Homepage:

• Replace the term homepage with dashboard (!96579 - merged)
• feat(nav): Replace term for GitLab Start Page (gitlab-org/gitlab-services/design.gitlab.com!3035 - merged)

Change to homepage

Set the docs and the user setting for "Dashboard" back to Homepage in the title and help instructions.