PEP 675: Add link to official discussion thread

[CAM-Gerlach]

It looks like PEP 675 hasn't yet been updated with the link to the latest official discussion thread (potentially contributing to the uncertainty of the community member on #2282 as to where to submit substantive feedback), which I believe I found after a fair amount of digging and a bit of guesswork. As such, this PR does just that.

CC: @pradeep90

[JelleZijlstra]

Existing PEPs are inconsistent, but this does seem more informative:

% grep 'Discussions-To.*yping' *.rst
pep-0585.rst:Discussions-To: Typing-Sig <typing-sig@python.org>
pep-0586.rst:Discussions-To: Typing-Sig <typing-sig@python.org>
pep-0589.rst:Discussions-To: typing-sig@python.org
pep-0591.rst:Discussions-To: typing-sig@python.org
pep-0593.rst:Discussions-To: typing-sig@python.org
pep-0604.rst:Discussions-To: typing-sig@python.org
pep-0612.rst:Discussions-To: Typing-Sig <typing-sig@python.org>
pep-0613.rst:Discussions-To: https://mail.python.org/archives/list/typing-sig@python.org/thread/MWRJOBEEEMFVXE7CAKO7B4P46IPM4AN3/
pep-0647.rst:Discussions-To: Typing-Sig <typing-sig@python.org>
pep-0655.rst:Discussions-To: typing-sig at python.org
pep-0673.rst:Discussions-To: Typing-Sig <typing-sig@python.org>
pep-0675.rst:Discussions-To: Typing-Sig <typing-sig@python.org>
pep-0681.rst:Discussions-To: https://mail.python.org/archives/list/typing-sig@python.org/thread/EAALIHA3XEDFDNG2NRXTI3ERFPAD65Z4/

[CAM-Gerlach]

@JelleZijlstra Just FYI, as discussed and agreed in #2266 , PEP 1 will be formally revised to clarify that direct links to the actual discussion thread should be preferred whenever possible over a generic, relatively unhelpful email address; its just pending merge of #2302 to avoid any merge conflicts before I go ahead and implement what we talked about. That won't necessarily involve mass-updating all old PEPs (though there would be tangible benefits to doing so), but will at least apply to any new PEPs and when current draft, active, etc. PEPs are updated.

As an aside, speaking from personal experience, this is one of the single biggest consistent usability barriers that I've experienced as a PEP reader and contributor, as well as likely contributed to the community member's uncertainty about where to post feedback on the PEP that we saw on #2282 .