docs: add gitlab and auth0 (#3700)

* docs: add gitlab and auth0

* Apply suggestions from code review

* fix wrong domain

* Apply suggestions from code review

Co-authored-by: mffap <mpa@zitadel.com>

* fix redirect-uris

* typos

* replace image wip

* smaller typos

* Update docs/docs/guides/basics/applications.mdx

Co-authored-by: mffap <mpa@zitadel.com>

* Update docs/docs/guides/basics/applications.mdx

Co-authored-by: mffap <mpa@zitadel.com>

Co-authored-by: Fabi <38692350+hifabienne@users.noreply.github.com>
Co-authored-by: mffap <mpa@zitadel.com>
Co-authored-by: Maximilian Panne <mpa@caos.ch>
Co-authored-by: Livio Spring <livio.a@gmail.com>

docs/sidebars.js

@@ -46,6 +46,7 @@ module.exports = {
         "guides/basics/instance",
         "guides/basics/organizations",
         "guides/basics/projects",
+        "guides/basics/applications",
       ],
     },
     {
@@ -54,6 +55,8 @@ module.exports = {
       collapsed: false,
       items: [
         "guides/integrations/authenticated-mongodb-charts",
+        "guides/integrations/auth0",
+        "guides/integrations/gitlab-self-hosted",
       ],
     },
     {