Add Vitepress docs (#326)

* Add vitepress for docs

* some fix

* Use vitepress and update docs

* Use vitepress and update docs

* Update README.md

* Add favicon

* Add docs by @jiriks74

Co-authored-by: jiriks74 <jiri@stefka.eu>

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Co-authored-by: jiriks74 <jiri@stefka.eu>

docs/configuration/admin-panel.md

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+# Admin panel
+
+The first user created on your Opengist instance has access to the Admin panel.
+
+To access the Admin panel:
+
+1. Log in
+2. Click your username in the upper right corner
+3. Select `Admin`
+
+## Usage
+
+### General
+
+Here you can see some basic information, like Opengist version, alongside some stats.
+
+You can also start some actions like forcing synchronization of gists,
+starting garbage collection, etc.
+
+### Users
+
+Here you can see your users and delete them.
+
+### Gists
+
+Here you can see all the gists and some basic information about them. You also have an option
+to delete them.
+
+
+### Invitations
+
+Here you can create invitation links with some options like limiting the number of signed up
+users or setting an expiration date.
+
+> [!Note]
+> Invitation links override the `Disable signup` option but not the `Disable login form` option.
+>
+> Users will see only the OAuth providers when `Disable login form` is enabled.
+
+### Configuration
+
+Here you can change a limited number of settings without restarting the instance.
+
+- Disable signup
+    - Forbid the creation of new accounts.
+- Require login
+    - Enforce users to be logged in to see gists.
+- Allow individual gists without login
+    - Allow individual gists to be viewed and downloaded without login, while requiring login for discovering gists.
+- Disable login form
+    - Forbid logging in via the login form to force using OAuth providers instead.
+- Disable Gravatar
+    - Disable the usage of Gravatar as an avatar provider.