Added ExceptionGroups section, intro sections, various fixes.

* move towards a PEP structure (according the PEP-12 headings). Added intro sections

* added section with ExceptionGroup spec

* Update except_star.md

Co-authored-by: Guido van Rossum <guido@python.org>

* Update except_star.md

Co-authored-by: Guido van Rossum <guido@python.org>

* Update except_star.md

Co-authored-by: Yury Selivanov <yury@edgedb.com>

* Update except_star.md

Co-authored-by: Guido van Rossum <guido@python.org>

* Update except_star.md

Co-authored-by: Guido van Rossum <guido@python.org>

* limit lines to 79 chars

* refined following reviews

* added para on catch to 'rejected ideas'

* s/ which,/, which/

* edited rejected ideas section following review

* updates to the old text (ExceptionGroup constructor not vararg, EG not iterable, raise and reraise not giving the same thing)

Co-authored-by: Guido van Rossum <guido@python.org>
Co-authored-by: Yury Selivanov <yury@edgedb.com>

Author: 3 people