Add individual documentation pages for string's subcommands

This adds string-x.rst for each subcommand x of string. The main page
(string.rst) is not changed, except that examples are shown directly after
each subcommand.  The subcommand sections in string.rst are created by
textual inclusion of parts of the string-x.rst files.

Subcommand man pages can be viewed with either of:

```
man string collect
man string-collect
string collect <press F1 or Alt-h>
string collect -h
```

While `string -h ...` still prints the full help.

Closes #5968

sphinx_doc_src/index.rst

@@ -730,7 +730,7 @@ Command substitution
 
 The output of a series of commands can be used as the parameters to another command. If a parameter contains a set of parenthesis, the text enclosed by the parenthesis will be interpreted as a list of commands. On expansion, this list is executed, and substituted by the output. If the output is more than one line long, each line will be expanded to a new parameter. Setting ``IFS`` to the empty string will disable line splitting.
 
-If the output is piped to :ref:`string split <cmd-string-split>` or `string split0 <cmd-string-split0>` as the last step, those splits are used as they appear and no additional splitting on newlines takes place.
+If the output is piped to :ref:`string split or string split0 <cmd-string-split>` as the last step, those splits are used as they appear and no additional splitting on newlines takes place.
 
 The exit status of the last run command substitution is available in the `status <#variables-status>`_ variable if the substitution occurs in the context of a ``set`` command.