Examples metadata in Cargo.toml (bevyengine#4741)

# Objective

- Have information about examples only in one place that can be used for the repo and for the website (and remove the need to keep a list of example to build for wasm in the website https://github.com/bevyengine/bevy-website/blob/75acb730406ef9c5928d37daf8bb32e4dbeb8b13/generate-wasm-examples/generate_wasm_examples.sh#L92-L99)

## Solution

- Add metadata about examples in `Cargo.toml`
- Build the `examples/README.md` from a template using those metadata. I used tera as the template engine to use the same tech as the website.
- Make CI fail if an example is missing metadata, or if the readme file needs to be updated (the command to update it is displayed in the failed step in CI)

## Remaining To Do

- After the next release with this merged in, the website will be able to be updated to use those metadata too
- I would like to build the examples in wasm and make them available at http://dev-docs.bevyengine.org/ but that will require more design
- bevyengine/bevy-website#299 for other ToDos

Co-authored-by: Readme <github-actions@github.com>

.github/workflows/ci.yml

@@ -162,7 +162,7 @@ jobs:
 
   markdownlint:
     runs-on: ubuntu-latest
-    needs: check-missing-examples-in-docs
+    needs: check-examples-readme-update-needed
     if: always()
     steps:
       - uses: actions/checkout@v3
@@ -261,14 +261,19 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-      - name: Check that examples are present in both README and Cargo
-        uses: Weibye/action-internal-link-consistency@1.0.0
-        with:
-          source: './examples/'
-          targets: '[ "./Cargo.toml", "./examples/README.md" ]'
-          file-types: '[".rs"]'
-          exclude-folders: '["./examples/ios"]'
-          exclude-files: '[]'
+      - run: cargo run -p build-example-pages -- check-missing
+
+  check-examples-readme-update-needed:
+    needs: check-missing-examples-in-docs
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - run: cargo run -p build-example-pages -- update
+      - name: Check for modified files
+        run: |
+          echo "if this step fails, run the following command and commit the changed file on your PR."
+          echo " > cargo run -p build-example-pages -- update"
+          git diff --quiet HEAD --
 
   check-unused-dependencies:
     runs-on: ubuntu-latest