feat(docs): add forge, prompts, tools, and settings documentation

Add setup guides for GitHub and GitLab forge integration, custom
prompts documentation, tools configuration guide, and settings
reference. Add design document for custom prompts and tools RFC.
Add webhook testing and configuration validation scripts. Update
README with forge integration features and configuration sections.

Signed-off-by: derekbarbosa <derekasobrab@gmail.com>

Author: derekbarbosa

README.md

@@ -19,10 +19,11 @@ Please, note that as with any other LLM-based tools, Sashiko's output is probabi
 
 ## Features
 
-- **Automated Ingestion**: Monitors mailing lists (using `lore.kernel.org`) for new patch submissions.
-- **Manual Ingestion**: Can ingest patches from a local git repository.
-- **Self-contained**: Doesn't depend on 3rd-party tools and can work with various LLM providers (Gemini and Claude are currently supported).
-- **Web interface and CLI**: Provides a web interface and a CLI tool. Email support will be added soon.
+- **Automated Ingestion**: Monitors mailing lists (`lore.kernel.org`), GitHub PRs, and GitLab MRs for new patch submissions.
+- **Manual Ingestion**: Can ingest patches from local git repositories or specific PRs/MRs.
+- **Forge Integration**: Automatic PR/MR review via GitHub and GitLab webhooks.
+- **Self-contained**: Doesn't depend on 3rd-party tools and works with multiple LLM providers (Gemini and Claude currently supported).
+- **Web interface and CLI**: Provides a web interface for monitoring and a CLI tool for local development.
 
 ## Prompts
 
@@ -90,6 +91,10 @@ cargo install sashiko
     *   **Server**: API server host and port.
     *   **Git**: Path to the reference kernel repository.
     *   **Review**: Concurrency and worktree settings.
+    *   **Tools**: Configure which AI tools are enabled (optional). See [docs/TOOLS.md](docs/TOOLS.md) for details.
+    *   **Forge**: GitHub/GitLab webhook integration (optional). See forge setup guides below.
+    *   **Prompts**: Customize review stages and prompts (optional). See [docs/PROMPTS.md](docs/PROMPTS.md) for details.
+    *   **Subsystems**: Map file patterns to subsystems for targeted reviews (optional).
 
     ### Configuring the LLM Provider