[3.14] gh-135676: Reword the f-string (and t-string) section (GH-137469) (GH-142227)

[3.14] gh-135676: Reword the f-string (and t-string) section (GH-137469) (GH-142227)

Much of the information was duplicated in stdtypes.rst; this PR keeps
lexical/syntactical details in Lexical Analysis and the evaluation & runtime
behaviour in Standard types, with cross-references between the two.
Since the t-string section only listed differences from f-strings, and the
grammar for the two is equivalent, that section was moved to Standard types
almost entirely.
(cherry picked from commit aea5531583aaa8bfdf3ebca914e9c694617c3489)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Blaise Pabon <blaise@gmail.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>