doc: Add changelog files.

Author: Hywan

crates/matrix-sdk-base/changelog.d/6893.added.md

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+We welcome the new `ThreadInfo` type! It is similar to `RoomInfo`, but tailored
+to threads!
+
+This type is managed by the Event Cache (`matrix_sdk::event_cache`).
+Consequently, the `EventCacheStore` trait gains 2 new methods:
+
+1. `load_thread_info` to load a `ThreadInfo`,
+2. `update_thread_info` to update a `ThreadInfo`.

crates/matrix-sdk-base/changelog.d/6893.changed.md

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+The `EventCacheStore::remember_thread` method has been removed.

crates/matrix-sdk/changelog.d/6893.added.md

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+`ThreadEventCache` gains 4 new methods:
+
+1. `read_receipts` to get the `ReadReceipts` associated to the `ThreadInfo`,
+2. `num_unread_messages` to get the number of unread messages (alias of
+   `read_receipts().await?.num_unread`),
+3. `num_unread_notifications` to get the number of unread notifications (alias
+   of `read_receipts().await?.num_notifications`),
+4. `num_unread_mentions` to get the number of unread mentions (alias of
+   `read_receipts().await?.num_mentions`).