[DOCS][Dev] Add Cache

docs/developer/src/cache.md

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+Cache
+=====
+
+In the Database chapter you've learned how
+GNU social allows you to store data in the
+database. Depending on your server's
+specification, the database can be a bottleneck. 
+To mitigate that, you can make use of an in-memory
+data structure storage to [cache](https://en.wikipedia.org/wiki/Cache_(computing))
+previous database requests. Using it is a great way of
+making GNU social run quicker. GNU social supports many
+adapters to different storages.
+
+Although different cache adapters provide
+different functionalities that could be nice
+to take advantage of, we had to limit our cache
+interface to the basic avaiable in all of them.
+I.e., _store_ and _delete_ operations.
+
+Store
+-----
+
+```php
+/**
+ * Get the cached avatar file info associated with the given GSActor id
+ *
+ * Returns the avatar file's hash, mimetype, title and path.
+ * Ensures exactly one cached value exists
+ */
+public static function getAvatarFileInfo(int $gsactor_id): array
+{
+    try {
+        $res = GSFile::error(NoAvatarException::class,
+            $gsactor_id,
+            Cache::get("avatar-file-info-{$gsactor_id}",
+                function () use ($gsactor_id) {
+                    return DB::dql('select f.file_hash, f.mimetype, f.title ' .
+                        'from App\Entity\Attachment f ' .
+                        'join App\Entity\Avatar a with f.id = a.attachment_id ' .
+                        'where a.gsactor_id = :gsactor_id',
+                        ['gsactor_id' => $gsactor_id]);
+                }));
+        $res['file_path'] = \App\Entity\Avatar::getFilePathStatic($res['file_hash']);
+        return $res;
+    } catch (Exception $e) {
+        $filepath = INSTALLDIR . '/public/assets/default-avatar.svg';
+        return ['file_path' => $filepath, 'mimetype' => 'image/svg+xml', 'title' => null];
+    }
+}
+```
+
+Delete
+------
+
+```php
+Cache::delete('avatar-file-info-' . $gsactor_id);
+```