[Context7] Support for git submodules and symlinked documentation in context7.json

### Problem Description We maintain a documentation monorepo that aggregates docs from multiple product repos (CLI, SDK, API, etc.) via symlinks. The actual markdown files live in each product repo but are symlinked into our main docs repo for unified publishing. Currently, Context7 doesn't follow symlinks or git submodules during indexing. This means: 1. The main docs repo gets indexed without the symlinked content 2. We need to register 4+ separate repos as individual Context7 projects 3. MCP queries require searching across multiple library IDs (`use context7 for org/docs, org/cli, org/sdk, org/api`) 4. Context is fragmented — the LLM doesn't see the docs as a unified whole ### Proposed Solution Add support in `context7.json` for including external sources, either via: **Option A: Submodule/symlink resolution** ```json { "followSymlinks": true, "includeSubmodules": true } ``` **Option B: Explicit external sources** ```json { "externalSources": [ { "repo": "org/cli",