I don't think Joomla does have the ability to do the 'shared' logins since some of those you mention (extensions, community, developer etc.) all seem to be different sub-sites (and treated as separate sites), all requiring separate registrations and logins.
However, two of those you mention do not use Joomla:
- The support forum.joomla.org uses PHPBB.
- The documentation site, docs.joomla.org uses MediaWiki
It's probably not easy "bridging" joomla and third-party applications like PhpBB or MediaWiki with a common login unless they all used social logins like Twitter, Facebook, Instagram etc.
I suppose the forum could use Kunena or similar but if documentation was Joomla articles instead of MediaWiki, that would be putting too tight a control on who can create the documentation content whereas with MediaWiki this is relaxed somewhat in the fact there is no 'approval' from a third party as such (as there is with Joomla articles - i.e. Publisher group users). Therefore anyone can create documentation content, and anyone can edit it.
Using social media logins across all areas seems sensible, but perhaps there are disadvantages to that as well.