Clean Contributors doc by splitting original huge single page into ni…

…ce chapters, and various related doc MD format fixes (google#2641)

docs/contrib/codespaces.md

@@ -16,3 +16,5 @@ Your contributions to make this better are very welcome.
 If you are missing your fancy custom Shell configuration that you have built over the last 100 years,
 you should [set up your dotfiles for Codespaces](https://docs.github.com/en/codespaces/setting-your-user-preferences/personalizing-github-codespaces-for-your-account#dotfiles)
 such as [e.g. this dude did in his dotfiles](https://github.com/vorburger/vorburger-dotfiles-bin-etc#github-codespaces).
+
+Gradle may fail with an _out-of-memory_ related error on its very first run; just re-run it once, and it should work in the Codespace.

docs/contrib/db.md

@@ -0,0 +1,17 @@
+# Database
+
+## Migrations
+
+If you are making changes to the database schema (in the `engine` or the `knowledge` module), you
+need to consider how applications with Android FHIR SDK dependencies can upgrade to the new schema
+without losing or corrupting existing data already on device. This can be done with [Room database
+migration](https://developer.android.com/training/data-storage/room/migrating-db-versions).
+
+!!! tip
+    A new JSON schema file will be generated under the `schemas` folder in the module when you
+    update the database version. If you are having trouble with this, make sure you run the gradle
+    command with `--rerun-tasks`:
+
+    ```sh
+    ./gradlew :<module>:build --rerun-tasks
+    ```