Add more content from the Interop Proposers Guide to the focus area proposal template

See this discussion for context: https://bsky.app/profile/romainmenke.bsky.social/post/3muopgir3nk2v
Some interesting bits from it:

An unspoken "rule" with the interop proposal is that it only seems to influence priority and not whether something will be implemented.

Being upfront and clear about this will avoid some frustration :)

In previous years some were frustrated because they felt they had made a strong case for X or Y and it still got rejected.
The bot reply leaves it up to the reader to interpret the various data points.

Simply stating up front that positive signals from all vendors is a requirement would help I think.

When people propose focus areas, they use a template which guides them. It's nice, and it even links to the Interop Proposers Guide.

But, based on the discussion mentioned above, it seems like we could improve things a little bit. Maybe all it takes is copy over some of the relevant bits from the Proposers Guide into the issue template, to manage expectations.

I imagine that not very many people actually read the guide. So few people will see this part, for example:

Proposals covering features where individual participating organizations on the Interop team have already indicated significant concerns, for example through standards-position repositories or similar public venues, are highly unlikely to be accepted

If that part (and other such "rules") were in the template, that might help with managing expectations.

Compare the focus-area proposal template at .github/ISSUE_TEMPLATE/focus-area-proposal.yml with proposal_guide.md, using the linked discussion for context. Identify the guide's relevant acceptance expectations and add them to the template so proposers understand how vendor signals affect proposals and decisions.