Move plugin docs to plugin folders

Summary: Moved plugin documentation and related assets to plugin folders, fixed links and configured redirects where required. Now these docs are used for both showing docs in Flipper and generating Flipper docs website.

Reviewed By: passy

Differential Revision: D29465567

fbshipit-source-id: 3ec4240b215b0d5baea5154f64266a9ba7ead3a5

desktop/plugins/public/databases/docs/overview.mdx

@@ -0,0 +1,14 @@
+import useBaseUrl from '@docusaurus/useBaseUrl';
+
+The Databases plugin provides developers with read-write access to their databases:
+- browse your tables' data
+- see your tables' structure
+- execute queries
+- mark queries as favorites
+- see logs of past executed queries
+
+
+<img alt="Databases Plugin 1" src={useBaseUrl("img/databases-plugin-1.png")} />
+<img alt="Databases Plugin 2" src={useBaseUrl("img/databases-plugin-2.png")} />
+
+Note: this plugin is only available for Android.