Turn off public sign up

## Problem

Some projects are invite-only and the AuthUI should not show the signup page. User can only login.
For example, access is only open to members or employees

This is supported in the backend but need to expose the config in the portal.

## Appetite

1 day

## Design

* Add a "Sign Ups" page under "Authentication"
* Create a page for Sign Up options. Currently only have 1 option

```markdown
# Sign Ups

Enable or Disable self-signup in this project

Allow users to sign up
[Toggle]
Anyone can signup to an account. Switch off to make it invite-only.
```

## Rabbit Hole

New users created from Social/Enterprise login should not be allowed too.

## Todo

- [ ] Add button in Authgear Portal
- [ ] Write a small doc page about "Disable public signup"

Start in the Authgear Portal's Authentication settings and trace the existing backend support for signup control. Add the Sign Ups page and self-signup toggle, then write the requested "Disable public signup" documentation; done when invite-only projects no longer show public signup and the setting is documented.

Written by the indexing model from the issue text.