[SHMEM 1.6 Sec 1-8] Updates and additions to front matter

[ronawho]

Summary of changes

This adds new 1.6 features to the front matter overview list and constant table and cleans up a few other areas. Specifically this:

• Adds new routines to the overview list and constant table for sessions, new signaling operations, and scan
• Updates the collectives overview to avoid talking about active-set specific behavior
• Tries to make the atomic overview language more consistent and adds missing routines
• Specifies that allocation routines are collective to match changes in the memory management section
• Removes deprecated functions from amo example 3 (sum_to_all -> reduce)
• Adds initial list of 1.6 contributors

Proposal Checklist

• [ ] Link to issue(s)
• [ ] Changelog entry
• [ ] Reviewed for changes to front matter
• [ ] Reviewed for changes to back matter