gh-135676: Reword the f-string (and t-string) section (GH-137469)

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.

Co-authored-by: Blaise Pabon <[email protected]>
Co-authored-by: Adam Turner <[email protected]>
Co-authored-by: Stan Ulbrych <[email protected]>