doc: New chapter "Strings and Characters"

[Bruno Haible]

This patch adds a new chapter "Strings and Characters" to the Gnulib
documentation.

I've updated the published doc, here:
https://www.gnu.org/software/gnulib/manual/gnulib.html#Strings-and-Characters

Please read, as it contains important info about the char32_t type and
32-bit wide character strings.


2023-05-15  Bruno Haible  <bruno@clisp.org>

        doc: New chapter "Strings and Characters".
        * doc/strings.texi: New file.
        * doc/gnulib.texi (POSIXURL): New variable.
        (posixheader, posixfunc, func): New macros, from GNU libunistring's
        documentation.
        Include strings.texi.
        (Particular Modules): Don't include c-locale.texi here.
        * doc/c-locale.texi: Sections become subsections, subsections become
        subsubsections.
        * doc/posix-functions/isalnum.texi: Mention c32isalnum.
        * doc/posix-functions/isalpha.texi: Mention c32isalpha.
        * doc/posix-functions/isblank.texi: Mention c32isblank.
        * doc/posix-functions/iscntrl.texi: Mention c32iscntrl.
        * doc/posix-functions/isdigit.texi: Mention c32isdigit.
        * doc/posix-functions/isgraph.texi: Mention c32isgraph.
        * doc/posix-functions/islower.texi: Mention c32islower.
        * doc/posix-functions/isprint.texi: Mention c32isprint.
        * doc/posix-functions/ispunct.texi: Mention c32ispunct.
        * doc/posix-functions/isspace.texi: Mention c32isspace.
        * doc/posix-functions/isupper.texi: Mention c32isupper.
        * doc/posix-functions/isxdigit.texi: Mention c32isxdigit.
        * doc/posix-functions/tolower.texi: Mention alternative APIs.
        * doc/posix-functions/toupper.texi: Likewise.
        * doc/posix-functions/towlower.texi: Mention c32tolower.
        * doc/posix-functions/towupper.texi: Mention c32toupper.
        * doc/posix-functions/wcswidth.texi: Mention c32swidth.
        * doc/posix-functions/wcwidth.texi: Mention c32width.

0001-doc-New-chapter-Strings-and-Characters.patch
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