Improve documentation for route maps

• Documents route map for Pages

From @markpundsack: I Google’d for GitLab route map and results were weird. Info is here: https://docs.gitlab.com/ce/ci/environments.html#go-directly-from-source-files-to-public-pages-on-the-environment, but hard to spot. I was expecting a section header saying route map, at least.

New doc or update?

• New documentation
• Update existing documentation

Checklists

Product Manager

• Add the correct labels
• Add the correct milestone
• Indicate the correct document/directory for this feature
• Fill the doc blurb below

Documentation blurb

• Doc title

• Feature overview/description

• Feature use cases

Developer

• Copy the doc blurb above and paste it into the doc
• Write the tutorial - explain how to use the feature
• Submit the MR using the appropriate MR description template