Add docs for Insiders Mode

[tommaso-moro]

Summary

This PR does two things

• I added a new insiders-features.md doc in docs/, which provides in-depth guidance on Insiders Mode + an overview of the features that will be enabled when users are on Insiders mode
• I updated the server-configuration.md guide to include information on how to configure the server to use Insiders mode

Why

• for consistency with our other config options which are all described in server-configuration.md
• for clarity as to what Insiders is, how to use it, and what functionality it adds
• for clarity on which hosts support MCP Apps for our server: vscode and vscode insiders

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)

[Copilot]

Pull request overview

Adds documentation for “Insiders Mode” to the GitHub MCP Server docs, describing how to enable it and what experimental features it unlocks (notably MCP Apps UIs).

Changes:

• Documented Insiders Mode configuration in docs/server-configuration.md (remote + local examples).
• Added a new docs/insiders-features.md page describing Insiders-only features and supported MCP Apps tools.
• Updated README.md to link to the new Insiders features documentation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
docs/server-configuration.md	Adds Insiders Mode to the quick reference table and includes a full configuration section with examples.
docs/insiders-features.md	New doc explaining Insiders Mode and listing MCP Apps-supported tools and client requirements.
README.md	Adds a link to the new Insiders features doc from the existing Insiders Mode section.