[feature] Add fields introduced in Mastodon 4.4.0 to /api/v2/instance response (#4240)

# Description

## Checklist

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [ ] 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.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [ ] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [ ] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4240
Co-authored-by: nicole mikołajczyk <git@mkljczk.pl>
Co-committed-by: nicole mikołajczyk <git@mkljczk.pl>

internal/api/model/instancev2.go

@@ -150,6 +150,15 @@ type InstanceV2URLs struct {
 	// Websockets address for status and notification streaming.
 	// example: wss://example.org
 	Streaming string `json:"streaming"`
+	// Address of the server about page.
+	// example: https://example.org/about
+	About string `json:"about"`
+	// Address of the server privacy policy, if any.
+	// example: https://example.org/about#rules
+	PrivacyPolicy *string `json:"privacy_policy"`
+	// Address of the server current terms of service, if any.
+	// example: https://example.org/about#privacy_policy
+	TermsOfService *string `json:"terms_of_service"`
 }
 
 // Hints related to translation.
@@ -199,6 +208,13 @@ type InstanceV2Registrations struct {
 	// Value will be null if no message is set.
 	// example: <p>Registrations are currently closed on example.org because of spam bots!</p>
 	Message *string `json:"message"`
+	// A minimum age required to register, if configured.
+	// Currently not implemented: will always be null.
+	MinAge *int `json:"min_age"`
+	// Whether registrations require the user to provide a reason for joining.
+	// Only applicable when ApprovalRequired is true.
+	// example: true
+	ReasonRequired bool `json:"reason_required"`
 }
 
 // Hints related to contacting a representative of the instance.