Add reason why the archive bot is joining the room

Using the join `reason` added in MSC2367

Related to adding some better profile information, #257 (comment)

docs/faq.md

@@ -17,6 +17,8 @@ And with the introduction of the jump to date API via
 [MSC3030](https://github.com/matrix-org/matrix-spec-proposals/pull/3030), we could show
 messages from any given date and day-by-day navigation.
 
+## Why did the archive bot join my room?
+
 ## How do I opt out and keep my room from being indexed by search engines?
 
 All public Matrix rooms are accessible to view in the Matrix Public Archive. But only

server/lib/matrix-utils/ensure-room-joined.js

@@ -43,6 +43,14 @@ async function ensureRoomJoined(
       method: 'POST',
       accessToken,
       abortSignal,
+      body: {
+        reason:
+          `Joining room to check history visibility. ` +
+          `If your room is public with shared or world readable history visibility, ` +
+          `it will be accessible at archive.matrix.org. ` +
+          `See the FAQ for more details: ` +
+          `https://github.com/matrix-org/matrix-public-archive/blob/main/docs/faq.md#why-did-the-archive-bot-join-my-room`,
+      },
     });
     assert(
       joinData.room_id,