Include a notice on local documentation indicating that it's being served locally

[jeremystretch]

Proposed Changes

When the NetBox documentation is being rendered for local use (i.e. the BUILD_PUBLIC environment variable is not set), include a subtle indication on the rendered page indicating that the documentation is being served locally.

Justification

Currently, the documentation includes the a meta tag when BUILD_PUBLIC is not set to True:

<meta name="robots" content="noindex">

This is used to disable search indexing for NetBox deployments which have been inadvertently exposed to the Internet (see #10496). However, the presence or omission of this tag is not easily detected. Adding a human-readable indication on the rendered page makes the distinction easier and conveys to the user that the documentation may differ from the current public release.