[wg/social] Participation: Move section from scope, Remove Chair Election, Add SWICG for Invited Experts

[plehegar]

This addresses the folllowing comments:

• "Workflow for Active Items" would probably be a better fit under "Participation" than under "Scope"
  that same subsection refers to "co-chair" elections, which I don't think are in line with the Process
• In the participation section, and given the strong influence of the SWICG, it seems odd that the CG is not mentioned in the paragraph about potential Invited Experts. The charter template suggests adding that, if there is a relevant CG.

cc w3c/strategy#435

[dmitrizagidulin]

@plehegar why the change to the Chairs policy? That wasn't requested by any reviewer in w3c/strategy#435, if I understand correctly, was it?