Adjust the title and introductory heading of the "Customers Portal Admin" page and add a table of contents.
This makes a few small improvements to https://about.gitlab.com/handbook/internal-docs/customers-admin/:
- It replaces the level-1 "Customers portal admin panel documentation" heading with a level-2 "Introduction" heading (the title is already presented in a level-1 heading).
- It appends "Panel Documentation" to the title (adapted from the previous introductory heading).
- It adds a table of contents.
cc @cynthia