Add documentation to validate phone numbers

docs/sessions/3pid.md

@@ -98,11 +98,17 @@ Built-in generators and connectors for supported 3PID types:
 
 ### Email
 Generators:
-- [Template](https://github.com/kamax-io/mxisd/blob/master/docs/threepids/email/notifications/template-generator.md)
+- [Template](https://github.com/kamax-io/mxisd/blob/master/docs/threepids/notifications/template-generator.md)
 
 Connectors:
-- [SMTP](https://github.com/kamax-io/mxisd/blob/master/docs/threepids/email/notifications/smtp-connector.md)
+- [SMTP](https://github.com/kamax-io/mxisd/blob/master/docs/threepids/medium/email/smtp-connector.md)
 
+#### MSISDN (Phone numbers)
+Generators:
+- [Template](https://github.com/kamax-io/mxisd/blob/master/docs/threepids/notifications/template-generator.md)
+
+Connectors:
+ - [Twilio](https://github.com/kamax-io/mxisd/blob/master/docs/threepids/medium/msisdn/twilio-connector.md) with SMS
 
 ## Usage
 ### Configuration
@@ -178,16 +184,6 @@ ID for each generator.
 In the above example, emails notifications are generated by the `example2` module and sent with the `example1` module.  
 By default, `template` is used as generator and `smtp` as connector.
 
-mxisd comes with the following IDs built-in:  
-**Connectors**
-- `smtp` for a basic SMTP connector, attempting STARTLS by default.
-See [the dedicated document](https://github.com/kamax-io/mxisd/tree/master/docs/threepids/email/notifications/smtp-connector.md)
-
-**Generators**
-- `template`, loading content from template files, using built-in mxisd templates by default.
-See [the dedicated document](https://github.com/kamax-io/mxisd/tree/master/docs/threepids/email/notifications/template-generator.md)
-for further configuration and customization options.
-
 ---
 
 `session.policy.validation` is the core configuration to control what users configured to use your Identity server