Add search/meilisearch documentation

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+# Configuring search
+
+{! backend/administration/CLI_tasks/general_cli_task_info.include !}
+
+## Built-in search
+
+To use built-in search that has no external dependencies, set the search module to `Pleroma.Activity`:
+
+> config :pleroma, Pleroma.Search, module: Pleroma.Activity
+
+While it has no external dependencies, it has problems with performance and relevancy.
+
+## Meilisearch
+
+To use [meilisearch](https://www.meilisearch.com/), set the search module to `Pleroma.Search.Meilisearch`:
+
+> config :pleroma, Pleroma.Search, module: Pleroma.Search.Meilisearch
+
+You then need to set the address of the meilisearch instance, and optionally the private key for authentication.
+
+> config :pleroma, Pleroma.Search.Meilisearch,
+>    url: "http://127.0.0.1:7700/",
+>    private_key: "private key"
+
+Information about setting up meilisearch can be found in the
+[official documentation](https://docs.meilisearch.com/learn/getting_started/installation.html).
+You probably want to start it with `MEILI_NO_ANALYTICS=true` and `MEILI_NO_CENTRY=true` environment variables,
+to disable analytics.
+
+### Private key authentication (optional)
+
+To set the private key, use the `MEILI_MASTER_KEY` environment variable when starting. After setting the _master key_,
+you have to get the _private key_, which is actually used for authentication.
+
+=== "OTP"
+    ```sh
+    ./bin/pleroma_ctl search.meilisearch show-private-key <your master key here>
+    ```
+
+=== "From Source"
+    ```sh
+    mix pleroma.search.meilisearch show-private-key <your master key here>
+    ```
+
+This is the key you actually put into your configuration file.
+
+### Initial indexing
+
+After setting up the configuration, you'll want to index all of your already existsing posts. Only public posts are indexed.  You'll only
+have to do it one time, but it might take a while, depending on the amount of posts your instance has seen. This is also a fairly RAM
+consuming process for `meilisearch`, and it will take a lot of RAM when running if you have a lot of posts (seems to be around 5G for ~1.2
+million posts while idle and up to 7G while indexing initially, but your experience may be different).
+
+To start te initial indexing, run the `index` command:
+
+=== "OTP"
+    ```sh
+    ./bin/pleroma_ctl search.meilisearch index
+    ```
+
+=== "From Source"
+    ```sh
+    mix pleroma.search.meilisearch index
+    ```
+
+This will show you the total amount of posts to index, and then show you the amount of posts indexed currently, until the numbers eventually
+become the same. The posts are indexed in big batches and meilisearch will take some time to actually index them, even after you have
+inserted all the posts into it. Depending on the amount of posts, this may be as long as several hours. To get information about the status
+of indexing and how many posts have actually been indexed, use the `stats` command:
+
+=== "OTP"
+    ```sh
+    ./bin/pleroma_ctl search.meilisearch stats
+    ```
+
+=== "From Source"
+    ```sh
+    mix pleroma.search.meilisearch stats
+    ```
+
+### Clearing the index
+
+In case you need to clear the index (for example, to re-index from scratch, if that needs to happen for some reason), you can
+use the `clear` command:
+
+=== "OTP"
+    ```sh
+    ./bin/pleroma_ctl search.meilisearch clear
+    ```
+
+=== "From Source"
+    ```sh
+    mix pleroma.search.meilisearch clear
+    ```
+
+This will clear **all** the posts from the search index. Note, that deleted posts are also removed from index by the instance itself, so
+there is no need to actually clear the whole index, unless you want **all** of it gone. That said, the index does not hold any information
+that cannot be re-created from the database, it should also generally be a lot smaller than the size of your database. Still, the size
+depends on the amount of text in posts.