feat: add a notion_append_block_content method that enables to write using Markdown

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LLMs struggle with very large tool schemas. Using this MCP server for relatively large Notion refactoring tasks, even the best LLMs today struggle using notion_append_block_children given the complexity of the Notion API. This PR introduces a new tool notion_append_block_content which enables the LLM to provide a Markdown text that will then be converted to Notion blocks by the server directly (using https://github.com/tryfabric/martian). It's much easier for LLMs to produce large chunk of contents this way.

@suekou let me know what you think. I did this on top of this MCP server because there is already a Markdown format support so it makes sense to me to support this both ways but I'd happily hear that you believe it's outside of the scope and would create another MCP server for it.

Lastly, going in this directly, I think that it would be worth removing the "notion to markdown" code from this repository and reuses something like https://github.com/souvikinator/notion-to-md instead.

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@suekou let me know, otherwise I'll fork away 🤓