Clarify the docs around open/closed source and public/private MCP servers (#573)

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## Motivation and Context
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The following PR adds more clarity to the docs around what's considered
open source, closed source, public and private in the context of MCP
servers.

## How Has This Been Tested?
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tested? -->

## Breaking Changes
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## Types of changes
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the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
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that apply. -->
- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
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Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

docs/guides/publishing/publish-server.md

@@ -19,6 +19,7 @@ By the end of this tutorial, you'll have:
 ## Prerequisites
 
 - An MCP server you've already built ([follow this guide if you don't have one already](https://modelcontextprotocol.io/quickstart/server))
+- Your server must be publicly accessible (closed source servers are welcome, but private/internal servers are not supported)
 
 ## Deployment Options