docs: describe frontend API tokens (#5784)

This change adds a mention of frontend API tokens to the docs. We've
had this functionality for a while, but it appears not to have been
documented.

Realized this as part of triaging #3005.

website/docs/using-unleash/deploy/configuring-unleash.md

@@ -67,7 +67,7 @@ unleash.start(unleashOptions);
     - `demo` - Only requires an email to sign in (was default in v3)
   - `customAuthHandler`: function `(app: any, config: IUnleashConfig): void` — custom express middleware handling authentication. Used when type is set to `custom`. Can not be set via environment variables.
   - `initialAdminUser`: `{ username: string, password: string} | null` — whether to create an admin user with default password - Defaults to using `admin` and `unleash4all` as the username and password. Can not be overridden by setting the `UNLEASH_DEFAULT_ADMIN_USERNAME` and `UNLEASH_DEFAULT_ADMIN_PASSWORD` environment variables.
-  - `initApiTokens` / `INIT_ADMIN_API_TOKENS` and `INIT_CLIENT_API_TOKENS` (see below): `ApiTokens[]` — Array of API tokens to create on startup. The tokens will only be created if the database doesn't already contain any API tokens. Example:
+    - `initApiTokens` / `INIT_ADMIN_API_TOKENS`, `INIT_CLIENT_API_TOKENS`, and `INIT_FRONTEND_API_TOKENS`: `ApiTokens[]` — Array of API tokens to create on startup. The tokens will only be created if the database doesn't already contain any API tokens. Example:
 
     ```ts
     [
@@ -83,7 +83,7 @@ unleash.start(unleashOptions);
 
     The tokens can be of any API token type. Note that _admin_ tokens **must** target all environments and projects (i.e. use `'*'` for `environments` and `project` and start the secret with `*:*.`).
 
-    You can use the environment variables `INIT_ADMIN_API_TOKENS` or `INIT_CLIENT_API_TOKENS` to create API admin or client tokens on startup. Both variables require a comma-separated list of API tokens to initialize (for instance `*:*.some-random-string, *:*.some-other-token`). The tokens found in `INIT_ADMIN_API_TOKENS` and `INIT_CLIENT_API_TOKENS` will be created as admin and client tokens respectively and Unleash will assign usernames automatically.
+    You can also use the environment variables `INIT_ADMIN_API_TOKENS`, `INIT_CLIENT_API_TOKENS`, and `INIT_FRONTEND_API_TOKENS`. All variables require a comma-separated list of API tokens to initialize (for instance `*:*.some-random-string, *:*.some-other-token`). The tokens found in `INIT_ADMIN_API_TOKENS`, `INIT_CLIENT_API_TOKENS`, and `INIT_FRONTEND_API_TOKENS` will be created as admin, client, and frontend tokens respectively, and Unleash will assign usernames automatically.
 
 - **databaseUrl** - (_deprecated_) the postgres database url to connect to. Only used if _db_ object is not specified, and overrides the _db_ object and any environment variables that change parts of it (like `DATABASE_SSL`). Should include username/password. This value may also be set via the `DATABASE_URL` environment variable. Alternatively, if you would like to read the database url from a file, you may set the `DATABASE_URL_FILE` environment variable with the full file path. The contents of the file must be the database url exactly.
 - **db** - The database configuration object. See [the database configuration section](#database-configuration) for a full overview of the available properties.