docs: 📝 Add docs about federation

app/federation/validation/page.mdx

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+export const metadata = {
+    title: 'Validation',
+    description:
+        'Validation rules for Lysand implementations.',
+}
+
+# Validation
+
+Implementations **MUST** strictly validate all incoming data to ensure that it is well-formed and adheres to the Lysand Protocol. If a request is invalid, the server **MUST** return a `400 Bad Request` HTTP status code.
+
+<Note>
+    Remember that while *your* implementation may disallow or restrict some user input, other implementations may not. You **should not** apply those restrictions to data coming from other instances.
+
+    For example, if your implementation disallows using HTML in posts, you should not strip HTML from posts coming from other instances. You *may* choose to display them differently, but you should not modify the data itself.
+</Note>
+
+Things that should be validated include, but are not limited to:
+
+- The presence of **all required fields**.
+- The **format** of all fields (integers should not be strings, dates should be in ISO 8601 format, etc.).
+- The presence of **all required headers**.
+- The presence of a **valid signature**.
+- The **length** of all fields (for example, the `username` field on a `User` entity should be at least 1 character long).
+  - Do not set arbitrary limits on the length of fields that other instances may send you. For example, a `bio` field should not be limited to 160 characters, even if your own implementation has such a limit.
+  - If you do set limits, they should be reasonable and well-documented.
+- The **type**, **precision** and **scale** of all numeric fields.
+  - For example, a `size` field on a `ContentFormat` structure should be a positive integer, not a negative number or a floating-point number.
+- The **validity** of all URLs and URIs (run them through your favorite URL parser).
+- The **time** of all dates and times (people should not be born in the future, or in the year 0).
+
+It is your implementation's duty to reject data from other instances that does not adhere to the strict spec. **This is crucial to ensure the integrity of your instance and the network as a whole**. Allowing data that is technically valid but semantically incorrect can lead to the degradation of the entire Lysand ecosystem.