Fortran manual: Revise introductory chapter.

Fortran manual: Revise introductory chapter.

Fix various bit-rot in the discussion of standards conformance, remove
material that is only of historical interest, copy-editing.  Also move
discussion of preprocessing out of the introductory chapter.

2021-11-01  Sandra Loosemore  <sandra@codesourcery.com>

	gcc/fortran/
	* gfortran.texi (About GNU Fortran): Consolidate material
	formerly in other sections.  Copy-editing.
	(Preprocessing and conditional compilation): Delete, moving
	most material to invoke.texi.
	(GNU Fortran and G77): Delete.
	(Project Status): Delete.
	(Standards): Update.
	(Fortran 95 status): Mention conditional compilation here.
	(Fortran 2003 status): Rewrite to mention the 1 missing feature
	instead of all the ones implemented.
	(Fortran 2008 status): Similarly for the 2 missing features.
	(Fortran 2018 status): Rewrite to reflect completion of TS29113
	feature support.
	* invoke.texi (Preprocessing Options): Move material formerly
	in introductory chapter here.