A bunch of our variables are only relevant for interactive use, but
this is two whole sections on them. Simply move them inside "Syntax
highlighting" and leave the link in Special Variables.
The file is called "config.fish", not "init.fish". We'll call it
"configuration" now.
"Initialization" might be slightly more precise, but in an irritating
way.
Also some wording improvements to the section. In particular we now
mention config.fish *early*, before the whole shebang.
fish_private_mode is active if set to something non-empty, but the docs
suggested checking if it is set at all. Switch the docs to match the
implementation through `test -n "$fish_private_mode"`
Fixes#8001
This breaks apart the massive "index" document into
1. An "index" document that explains how to install and set up fish
and links to the other documents
2. A "fish-language" document that describes the syntax and semantics
of the language
3. A "fish-interactive" document that describes how to use fish
interactively
No change to the content has been made, only the parts have been moved
from index and some of the formatting (links and header levels) were
fixed.
See #7348.
