docs: mandate public adopters list for growth stage projects

[dsterz]

I would like to propose adding a requirement for projects to publicly list their adopters, but I suggest we place this in the Growth stage rather than waiting until Graduation.

My thinking here is that bringing this transparency forward in the project lifecycle offers three major benefits for our ecosystem:

• Lowers Adoption Hesitation: Transparency of adoption is an essential trust signal for prospective users. By showcasing real-world use earlier, we can help eliminate the hesitation enterprises feel when evaluating new software.
• Increases Trust through Transparency: Publicly showcasing adoption proves that these are widely accepted standards, which encourages more providers and integrators to join and helps us circumvent market fragmentation.
• Enables Easier Open Collaboration: When it is public knowledge who is already using the project, it becomes much easier for maintainers to communicate with them. As we note in our foundation goals, real-world adoption and feedback raise the quality and reliability of the entire stack for everyone.

This places virtually no technical burden on developers but will significantly aid the TAC in proving the early success of our initiatives.

[jakobmoellerdev]

The "primary" repository is undefined. I believe since we anyhow request a web presence (https://github.com/neonephos/guidelines-development/pull/5/changes#diff-d581ac8f08d67c132fa72c8c07f2b40c58ad6d723a299c43784623b3a08b9256R57) we could make them force adoption there?

On top of this I believe we need to define very clearly what adoption means, and at which point a project counts as adopted (I believe you have something correlated in a separate pr here #4)

[fmui]

IMO, this should be a SHOULD requirement for all projects. We can define the specifics in the project guidelines to be more flexible. The policy may elevate it to a MUST for projects in the Growth and Graduated stages.