docs: add legacy password hashing

Author: wangsijie

docs/user-management/user-migration.mdx

@@ -20,7 +20,29 @@ You can create a map to match the user info from your existing user profile to *
 
 Logto uses [Argon2](https://en.wikipedia.org/wiki/Argon2) to hash the user's password, and also supports other algorithms like `MD5`, `SHA1`, `SHA256` and `Bcrypt` for the convenience of migration. Those algorithms are considered insecure, the corrosponding password hashes will be migrated to Argon2 upon the user's first successful sign in.
 
-If you need support for any particular hashing algorithm, [please let us know](https://logto.io/contact).
+If you are using other hashing algorithms or salt, you can set the `passwordAlgorithm` to `Legacy`, this allows you to use any hash algorithm supported by Node.js. In this case, the `passwordDigest` will be a JSON string that contains the hash algorithm and other algorithm-specific parameters.
+
+The format of the JSON string is as follows:
+
+```json
+["hash_algorithm", ["argument1", "argument2", ...], "expected_hashed_value"]
+```
+
+And you can use @ as the input password in the arguments.
+
+For example, if you are using SHA256 with a salt, you can store the password in the following format:
+
+```json
+["sha256", ["salt123", "@"], "c465f66c6ac481a7a17e9ed5b4e2e7e7288d892f12bf1c95c140901e9a70436e"]
+```
+
+This equals to the following code:
+
+```ts
+const hash = crypto.createHash('sha256');
+hash.update('salt123' + 'password123');
+const expectedHashedValue = hash.digest('hex');
+```
 
 ## Steps to migrate \{#steps-to-migrate}