website/integations/services: Slack integration docs (#9933)

* draft and sidebar

* added rest of config steps

* formatting

* typo

* tweaks

* contributors edit

* fixed support badge

* fix formatting

* marc edit

* 4d62 edit

* tech edits

* fix prettier error

* add link from Jens

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Co-authored-by: Tana M Berry <tana@goauthentik.com>

website/sidebarsIntegrations.js

@@ -40,6 +40,7 @@ module.exports = {
                         "services/rocketchat/index",
                         "services/roundcube/index",
                         "services/sharepoint-se/index",
+                        "services/slack/index",
                         "services/vikunja/index",
                         "services/wekan/index",
                         "services/wiki-js/index",