PEP 694: Address additional feedback (#4549)

* Begin to address William's feedback.

* Require that `name` conform to the normalization rules, and include a link
* Require that `version` conform to the version specs, and include a link
* RFC 3399 instead of ISO 8601 as the timestamp spec.  The RFC is a simpler format that subsets the
  ISO standard, and is more appropriate to our use case.
* Adjust the gentoken() algorithm to be more resistant to tomfoolery.  This may still change.
* Require `filename` to conform to either the source or binary distribution file name convention,
  and include links

* Further updates based on DPO thread

* The addition of the ``Location`` header is now a **MUST**, and better worded to indicate that it
  can be polled in the case of a ``202 Accepted``.
* Added a couple of **FIXME** tags to address removal of nonce and fleshing out the ``Errors``
  section.  See URLs in the text.  DO NOT PROMOTE FROM DRAFT UNTIL THIS IS DONE.
* Reformatted some text.

* Fix markup

* Remove all mention of the nonce

Based on discussions here:
https://discuss.python.org/t/pep-694-pypi-upload-api-2-0-round-2/101483/22 clients no longer supply
a nonce to influence the session token and stage URL.  The calculation of these is left to the
index, but language is added that if provided, they must be cryptographically unguessable, and it
must be possible to calculate the stage URL from the session token.

* Move some text around so it flows better

* Capitalization consistency

* Update the file upload session section

* capitalization

* Resolve some FIXMEs

* Clarifications based on comments from @mgorny

* When an attempt is made to create a second session with the same name-version pair, but the first session is
  in `pending`, `processing`, or `complete` state, the second session is *not* created and a 409 is returned.
* There's nothing special about `0.0.0` as a placeholder version, and it really should be `0.0.0a0`.

* add a TBD

* Define index-specific-metadata

* Add Change History section

* Add a change history section

Author: warsaw