update SIG joining

Summary

This PR updates the SIG directory to establish a lightweight, transparent process for joining an existing Special Interest Group.

Why

Previously, an individual could add themselves to a SIG by submitting a Pull Request, without providing information about their experience or receiving support from existing SIG members. Because listed SIG members may help review Development Fund proposals and provide technical or domain guidance, membership should include a basic level of community validation.

Changes

• Requires applicants to introduce themselves and describe their relevant experience and expected contribution in the PR.
• Requires one current SIG member to post a substantive recommendation.
• Requires a second current SIG member to endorse the applicant.
• Requires at least one endorser to be from an organization other than the applicant’s organization.
• Clarifies that discussion-channel access does not automatically confer formal SIG membership.
• Clarifies that SIG membership does not provide voting rights, automatic Champion status, funding preference, proposal approval authority, or authority to represent the Foundation.
• Requires Tech & Ops Voting Group approval before a new SIG may be created.
• Corrects existing Markdown formatting and removes duplicate roster entries.

This does not change the open nature of SIG discussions or require existing members to reapply. Current SIG rosters remain unchanged other than the removal of duplicate entries.