[3.14] gh-132661: Document t-strings and `templatelib` (GH-135229) (#136974)

Co-authored-by: Dave Peck <davepeck@gmail.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Loïc Simon <loic.pano@gmail.com>
Co-authored-by: pauleveritt <pauleveritt@me.com>
Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>