help: get section titles from Sphinx

functions/help and completions/help duplicate a lot of information
from doc_src. Get this information from Sphinx.

Drop short section titles such as "help globbing" in favor of the
full HTML anchor:

	help language#wildcards-globbing 

I think the verbosity is no big deal because we have tab completion,
we're trading in conciseness for consistency and better searchability.

In future, we can add back shorter invocations like "help globbing"
(especially given that completion descriptions often already repeated
the anchor path), but it should be checked by CI.

Also
- Remove some unused Sphinx anchors
- Remove an obsoleted script.
- Test that completions are in sync with Sphinx sources.
  (note that an alternative would be to check
  in the generated help_sections.rs file, see
  https://internals.rust-lang.org/t/how-fail-on-cargo-warning-warnings-from-build-rs/23590/5)

Here's a list of deleted msgids. Some of them were unused, for others
there was a better message (+ translation).

	$variable $variable 变量
	(command) command substitution (命令) 命令替换
	< and > redirections < 和 > 重定向
	Autoloading functions 自动加载函数
	Background jobs 后台作业
	Builtin commands 内建命令
	Combining different expansions 合并不同的展开
	Command substitution (SUBCOMMAND) 命令替换 (子命令)
	Defining aliases 定义别名
	Escaping characters 转义字符
	Help on how to reuse previously entered commands 关于如何重复使用先前输入的命令的帮助
	How lists combine 列表如何组合
	Job control 作业控制
	Local, global and universal scope 局域、全局和通用作用域
	Other features 其他功能
	Programmable prompt 可编程提示符
	Shell variable and function names Shell 变量和函数名
	Some common words 一些常用词
	The status variable 状况变量
	Variable scope for functions 函数的变量作用域
	Vi mode commands Vi 模式命令
	What set -x does `set -x` 做什么
	Writing your own completions 自己写补全
	ifs and elses if 和 else
	var[x..y] slices var[x..y] 切片
	{a,b} brace expansion {a,b} 大括号展开
	~ expansion ~ 展开


Closes #11796

doc_src/language.rst

@@ -41,8 +41,6 @@ Switches differ between commands and are usually documented on a command's manua
 
 So the basic idea of fish is the same as with other unix shells: It gets a commandline, runs :ref:`expansions <expand>`, and the result is then run as a command.
 
-.. _terminology:
-
 Terminology
 -----------
 
@@ -1085,8 +1083,6 @@ The ``~`` (tilde) character at the beginning of a parameter, followed by a usern
 
   echo ~root # prints root's home directory, probably "/root"
 
-.. _combine:
-
 Combining different expansions
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -1887,8 +1883,6 @@ For a list of all builtins, use ``builtin -n``.
 
 For a list of all builtins, functions and commands shipped with fish, see the :ref:`list of commands <Commands>`. The documentation is also available by using the ``--help`` switch.
 
-.. _command-lookup:
-
 Command lookup
 --------------
 
@@ -1967,8 +1961,6 @@ Let's make up an example. This function will :ref:`glob <expand-wildcard>` the f
 
 If you run this as ``show_files /``, it will most likely ask you until you press Y/y or N/n. If you run this as ``show_files / | cat``, it will print the files without asking. If you run this as ``show_files .``, it might print something without asking because there are fewer than five files.
 
-.. _identifiers:
-
 Shell variable and function names
 ---------------------------------