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/cmds/string-split0.rst

@@ -0,0 +1,5 @@
+string-split0 - split on zero bytes
+===================================
+
+.. include:: string-split.rst
+   :start-line: 2