Define docs nav link text and slug in front matter

[alanbernstein]

Currently the nav elements are defined in the doc file front matter like this:

nav = [
    "Installing in production",
    "Importing and Exporting Data",
    "Versioning",
    "Backup/restore",
]

where these values must match the header text. If we instead use a dict of slug: text, then custom link text can be used in the nav bar. Then headers can be as long as desired, without forcing the nav link text to overflow, since the link text can be shortened.

This will require updates both here and in each docs file.