Add reference documentation under the docs folder

[tzolov]

Add MCP Java SDK reference documentation under the docs folder. Use gh-pages to server deploy and serve the reference content.

[tzolov]

Resolved by #795

The reference documentation is maintained as mkdocs-material under the docs folder in main branch.
The docs.yml GH action is responsible to generate the static content served via gh-pages on https://modelcontextprotocol.github.io/java-sdk/
The API is currently served through javadoc.io on https://javadoc.io/doc/io.modelcontextprotocol.sdk