Unify group sites and templatize

[kimdhamilton]

For some reason, we created SIG and user group sites by forking github repos just to create 1 web page sml. This unifies all the SIG and UG files into the DIF web site and makes everything config driven, making it much easier to update the site when we add a group.

While likely not needed, I added redirects for the older SIG links

Other changes:

• Moved archived groups to a separate, less prominent section at the bottom, and don't include them in the nav menu
• Provide "overview" page for each type of group
• Reduced unneeded design variation in SIGs and user groups, but kept content and made formatting consistent
• Added general links to recordings, calendar, and meeting information: note this also solves the problem of stale calendar invites, old recording links, and consistency of display.

I will no doubt come back and clean up the (cursor-assisted) code generation steps, but this frees us from significant effort when we add a new group.