update slack url and twitter (#355)

* update slack url and twitter

use a dedicated url for slack, allowing update in a single place

rename Twitter references to X

* rollback Install NetBird title in navigation menu

src/pages/selfhosted/self-hosted-vs-cloud-netbird.mdx

@@ -64,5 +64,5 @@ your critical network infrastructure.
 
 - Make sure to [star us on GitHub](https://github.com/netbirdio/netbird)
 - Follow us [on X](https://x.com/netbird)
-- Join our [Slack Channel](https://join.slack.com/t/netbirdio/shared_invite/zt-31rofwmxc-27akKd0Le0vyRpBcwXkP0g)
+- Join our [Slack Channel](/slack-url)
 - NetBird [latest release](https://github.com/netbirdio/netbird/releases) on GitHub

src/pages/selfhosted/selfhosted-guide.mdx

@@ -241,8 +241,8 @@ To upgrade NetBird to the latest version, you need to review the [release notes]
 
 ## Get in touch
 
-Feel free to ping us on [Slack](https://join.slack.com/t/netbirdio/shared_invite/zt-31rofwmxc-27akKd0Le0vyRpBcwXkP0g) if you have any questions
+Feel free to ping us on [Slack](/slack-url) if you have any questions
 
 - NetBird managed version: [https://app.netbird.io](https://app.netbird.io)
 - Make sure to [star us on GitHub](https://github.com/netbirdio/netbird)
-- Follow us [on Twitter](https://twitter.com/netbird)
+- Follow us [on X](https://x.com/netbird)

src/pages/selfhosted/selfhosted-quickstart.mdx

@@ -111,8 +111,8 @@ rm -f docker-compose.yml Caddyfile zitadel.env dashboard.env machinekey/zitadel-
 
 ## Get in touch
 
-Feel free to ping us on [Slack](https://join.slack.com/t/netbirdio/shared_invite/zt-31rofwmxc-27akKd0Le0vyRpBcwXkP0g) if you have any questions
+Feel free to ping us on [Slack](/slack-url) if you have any questions
 
 - NetBird managed version: [https://app.netbird.io](https://app.netbird.io)
 - Make sure to [star us on GitHub](https://github.com/netbirdio/netbird)
-- Follow us [on Twitter](https://twitter.com/netbird)
+- Follow us [on X](https://x.com/netbird)