fixed broken links, added username restrictions, minor docs updates (#2580)

docs/source/01-01-talk-quickstart.md

@@ -10,7 +10,7 @@ smarter discussions around your work.
 More than 60 newsrooms use Talk to run their on-site communities, including the Washington Post, the Wall Street Journal, and IGN.
 
 [Read more about our product features and
-goals here](https://coralproject.net/talk).
+goals here](https://coralproject.net/).
 
 <div class="callout">
   We offer hosting and support packages for Talk. [Contact us for more information.](https://coralproject.net/pricing/)

docs/source/version-5-installation.md

@@ -6,7 +6,7 @@ permalink: /
 Online comments are broken. Our open-source commenting platform, Coral, rethinks
 how moderation, comment display, and conversation function, creating the
 opportunity for safer, smarter discussions around your work.
-[Read more about Coral here](https://coralproject.net/talk).
+[Read more about Coral here](https://coralproject.net/).
 
 Built with ❤️ by The Coral Project, a part of [Vox Media](https://product.voxmedia.com/).
 

docs/source/version-5-sso.md

@@ -39,7 +39,11 @@ You will then have to generate a JWT with the following claims:
   authentication system. This is required to facilitate notification email's
   about status changes on a user account such as bans or suspensions.
 - `user.username` (**required**) - the username that should be used when being
-  presented inside Coral to moderators and other users.
+  presented inside Coral to moderators and other users. The following restrictions apply :
+  - Can only contain letter (`a-zA-Z`), number (`0-9`), underscore (`_`) or period (`.`) characters. Spaces and other special characters are not allowed.
+  - Min length = 3
+  - Max length = 30
+
 - `user.badges` (_optional_) - array of strings to be displayed as badges beside
   username inside Coral, visible to other users and moderators. For example, to indicate
   a user's subscription status.