Add link to new security policy

[sethmlarson]

Adds a link to the new security policy available in the Developers Guide. cc @python/psrt

[sharktide]

skip-issue here

[sharktide]

I know we just merged this, but why did we remove the block on release versions? It could stop a lot of questions like "is Python 3.9 still receiving security updates?"

[sharktide]

cc @gpshead

[sethmlarson]

@sharktide Good question: we have that information within the security policy in this section: https://devguide.python.org/security/policy/#what-versions-of-python-accept-reports

Does that section cover this question completely? If not: happy to fix it up.

[sharktide]

It does, I was just thinking about being more explicit in the docs. I'm indifferent however, and I will leave it up to you and the core team to decide. :)

[gpshead]

lets see how things go report wise. that we have docs in two places and the devguide being a separate repo isn't available to every agent reading the repo is a known potential issue.