docs: add guide for edit docs and preview in local (#5005)

Author: leesb971204

CONTRIBUTING.md

@@ -23,6 +23,58 @@
   - `pnpm dev`
 - Make changes to the code
   - If you ran `pnpm dev` the dev watcher will automatically rebuild the code that has changed
-- Changes to the docs
-  - Be sure that internal links are written always relative to `docs` folder.
-    E.g. `./guide/data-loading`
+- Editing the docs locally and previewing the changes
+  - The documentations for all the TanStack projects are hosted on [tanstack.com](https://tanstack.com), which is a TanStack Start application (https://github.com/TanStack/tanstack.com). You need to run this app locally to preview your changes in the `TanStack/router` docs.
+
+> [!NOTE]
+> The website fetches the doc pages from GitHub in production, and searches for them at `../router/docs` in development. Your local clone of `TanStack/router` needs to be in the same directory as the local clone of `TanStack/tanstack.com`.
+
+You can follow these steps to set up the docs for local development:
+
+1. Make a new directory called `tanstack`.
+
+```sh
+mkdir tanstack
+```
+
+2. Enter that directory and clone the [`TanStack/router`](https://github.com/TanStack/router) and [`TanStack/tanstack.com`](https://github.com/TanStack/tanstack.com) repos.
+
+```sh
+cd tanstack
+git clone [email protected]:TanStack/router.git
+# We probably don't need all the branches and commit history
+# from the `tanstack.com` repo, so let's just create a shallow
+# clone of the latest version of the `main` branch.
+# Read more about shallow clones here:
+# https://github.blog/2020-12-21-get-up-to-speed-with-partial-clone-and-shallow-clone/#user-content-shallow-clones
+git clone [email protected]:TanStack/tanstack.com.git --depth=1 --single-branch --branch=main
+```
+
+> [!NOTE]
+> Your `tanstack` directory should look like this:
+>
+> ```
+> tanstack/
+>    |
+>    +-- router/ (<-- this directory cannot be called anything else!)
+>    |
+>    +-- tanstack.com/
+> ```
+
+3. Enter the `tanstack/tanstack.com` directory, install the dependencies and run the app in dev mode:
+
+```sh
+cd tanstack.com
+pnpm i
+# The app will run on https://localhost:3000 by default
+pnpm dev
+```
+
+4. Now you can visit http://localhost:3000/router/latest/docs/framework/react/overview in the browser and see the changes you make in `tanstack/router/docs` there.
+
+> [!WARNING]
+> You will need to update the `docs/(router or start)config.json` file (in `TanStack/router`) if you add a new documentation page!
+
+You can see the whole process in the screen capture below:
+
+https://github.com/fulopkovacs/form/assets/43729152/9d35a3c3-8153-4e74-9cb2-af275f7a269b