GNAT Project modes (`gpr-mode` and `gpr-ts-mode`) exist to handle .gpr files.
These modes are now used instead of the Ada major mode.
Lines may need to be re-indented when RET is pressed. This is to handle cases
of incomplete syntax and ambiguity in what may be entered when an empty line is
initially indented. Re-indenting after text has been entered corrects
incorrectly guessed initial indentation. To accommodate this scenario, RET is
remapped to `reindent-then-newline-and-indent`.
Also updates documentation to reflect these changes.
