gh-132661: Document t-strings and templatelib

[davepeck]

This PR adds documentation for t-strings, including:

• Document string.templatelib in the Library reference section
• Add new t-string details to lexical-analysis.rst
• Add AST nodes to the ast docs
• Add BUILD_INTERPOLATION and BUILD_TEMPLATE to the dis docs
• Update the PEP 292 section Template strings section of Doc/library/string.rst to clarify that this is unrelated to t-strings
• Add a t-strings section to "fancier input/output formatting", after the f-strings section
• Glossary

We also have an issue tracking our original list of PEP 750 documentation tasks.

• Issue: Implement PEP 750 – Template Strings #132661

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📚 Documentation preview 📚: https://cpython-previews--135229.org.readthedocs.build/