Add HA guide for self-hosted Enterprise (#812)

* render mermaid disgrams

* wip: add ha docs

* improve high availability guide

* fix review

* remove nav links and references

* remove dead server.authSecret references

* fix Redis troubleshooting command using non-existent env var

* correct startup-log order

package.json

@@ -53,6 +53,7 @@
     "lodash": "^4.17.21",
     "mdast-util-to-string": "^3.2.0",
     "mdx-annotations": "^0.1.1",
+    "mermaid": "^11.15.0",
     "next": "^16.0.0",
     "openapi-types": "^12.1.0",
     "postcss-focus-visible": "^8.0.2",