Add SIGs to Handbook

Why is this change being made?

This MR adds a concepts of Special Interest Groups (SIGs) to Handbook.

A Special Interest Group (SIG) is a group of people sharing similar interest, passion or deep expertise in the same concern / area.

Special Interest Groups are a concept widely known and used across the industry, including some of the most successful open source projects in history. SIGs have been most recently popularized by Kubernetes and are becoming a common governance model for Linux Foundation projects. SIGs have a rich history in other communities such as the Python Software Foundation, Fedora Project, and CentOS. Even though we recognize that this term is not perfect, there are notable benefits of using the same nomenclature like the one already recognized by the wider community.

SIGs at GitLab serve four main purposes:

1. Formally encode technical leadership into our organization, allowing experts to define and maintain a cohesive technical vision across functional areas.
2. Provide the resources and structure necessary to execute on cross-functional initiatives, ensuring we remain strategically aligned with the defined vision.
3. To better support our Product Team by providing technical guidance that is grounded in the long-term technical vision maintained by SIG members.
4. Engage more directly with community contributors and, in particular, the engineering teams of large customers who have unique insights into building platforms on top of GitLab and operating it at scale. This allows Everyone To Contribute towards the technical vision and direction maintained by SIGs.

This initiative has been started by @marshall007 and myself and this MR is a product of our discussions about it with GitLab team members. See #sigs channel on Slack (available only for team members).

You can find more details and the problem statement in the following Google Document (internal): https://docs.google.com/document/d/1bAnnpZdXkl63j_88ZZfB8D49Cpyhf0kAgNiTakuNNf4

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