Allow making non-native site links server-absolute

[QuLogic]

... instead of globally absolute with an explicit server name. This allows the docs to be served from a different server (e.g., on localhost), without links leaking out to the real matplotlib.org.

[jklymak]

The top level won't resolve for the other websites that use the theme, though. Ie mpl-third-party etc?

[QuLogic]

Locally maybe, but not once on the site. They are all on the same server. Maybe we need an option as in matplotlib/mpl-brochure-site#41?

[QuLogic]

OK, I modified the native_site option (which I never really understood the name of) into a navbar_links option, with 'absolute', 'relative', and 'internal' choices. This allows setting paths as https://matplotlib.org/..., /stable/..., or pathto('...'), and can be set by the theme config, or overridden by passing, e.g., -Dhtml_theme_options.navbar_links=internal.

[QuLogic]

Alternatively, perhaps this should be navbar_link_prefix, defaulting to https://matplotlib.org/stable/, can be overridden for the website with /stable, and setting None would mean fall back to pathto?

[tacaswell]

Discussed with Elliott on a a call, suggested this be rename to "server_stable" or something similar..

[QuLogic]

Done, and rebased.