[docs] Update docs to differentiate between Service, Person, and Application actor types, add FEDERATION.md linking to docs (#4566)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

Just realized our docs were out of date for the different actor types we use. This fixes that, and adds a FEDERATION.md document pointing to the online docs + markdown files in the repo.

Relates to https://codeberg.org/superseriousbusiness/gotosocial/issues/4565

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4566
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

FEDERATION.md

@@ -0,0 +1,5 @@
+# Federation
+
+Please see the [Federation](https://docs.gotosocial.org/en/latest/federation/) section of the online documentation.
+
+You can view the Markdown files that this documentation is built from [here](./docs/federation/).

docs/federation/actors.md

@@ -1,12 +1,14 @@
 # Actors and Actor Properties
 
-## `Service` vs `Person` actors
+## `Service` vs `Person` vs `Application` actors
 
 GoToSocial serves most accounts as the ActivityStreams `Person` type described [here](https://www.w3.org/TR/activitystreams-vocabulary/#dfn-person).
 
-Accounts that users have selected to mark as bot accounts, however, will use the `Service` type described [here](https://www.w3.org/TR/activitystreams-vocabulary/#dfn-service).
+Accounts that users have selected to mark as bot accounts, however, will use the `Application` type described [here](https://www.w3.org/TR/activitystreams-vocabulary/#dfn-application).
 
-This type distinction can be used by remote servers to distinguish between bot accounts and "regular" user accounts.
+The "instance service" account, which occasionally makes requests to remote endpoints on the instance's behalf, uses the `Service` type described [here](https://www.w3.org/TR/activitystreams-vocabulary/#dfn-service). Typically, remote instances will only see this actor making http-signed requests for media files.
+
+If desired, this type distinction can be used by remote servers to distinguish between "regular" user accounts and other types. Typically, this is done using a "bot" flag on the web and/or client API.
 
 ## Inbox