Add information about consortium memberships

Why is this change being made?

I've added a list of consortium memberships that the Community Relations team belongs to. This may be best suited living under the Community Relations handbook page, but I've added it here as a first iteration since the memberships are aligned most with our GitLab for Open Source program.

This section includes:

• Current membership -- List of current memberships and levels. Does not provide cost info.
• Evaluation criteria -- This should help organizations understand what kind of criteria we use to evaluate whether or not to join a membership. I'd like to keep improving this!
• Other sponsorship -- this is mainly about event sponsorship and leaves out any other discretionary sponsorships we may choose to engage in.

Related issues:

• Overview of memberships
• Evaluate strategic memberships

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

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