[Check-in] Add section for the Logout Endpoint (#549)

* Add section for Logout Endpoint
* Fix OIDC terminology
* Update instruction of creating SAML SP metadata
* Add section for Redirect URIs
* Add section for UserInfo Endpoint
* Add section for Introspection Endpoint
* Fix note references to the endpoints table
* Move grant flows from `Endpoints` to `Grant Types` section
* Make duplicate headers unique
* Fix natural language errors
* Add instruction for Authorization Code Flow
* Add section for `Device Authorization Endpoint`
* Add section for `JSON Web Key Sets Endpoint`
* Change language highlighter for HTTP outputs
* Add the `$` prompt to all shell commands
* Change syntax highlighter for NGINX config options

Co-authored-by: Baptiste Grenier <baptiste.grenier@egi.eu>