Api doc page still can be accessed when the ui is disabled

__Steps to reproduce:__
List the minimal actions needed to reproduce the behaviour.

1. set ui-disabled: true
2. restart influxdb
3. access http://x.x.x.x:8086/docs. The api doc page still can be accessed.

__Expected behaviour:__
When the ui is diabled, the api doc page should be disabled too.

__Actual behaviour:__
The api doc page still can be accessed.

__Environment info:__

* Use release package https://dl.influxdata.com/influxdb/releases/influxdb2-2.7.5_darwin_amd64.tar.gz
* Linux 3.10.0-1160.88.1.el7.x86_64 x86_64

__Config:__
ui-disabled: true

__Logs:__

Reproduce the issue by setting `ui-disabled: true`, restarting InfluxDB, and requesting `http://x.x.x.x:8086/docs`. Trace the handler for the `/docs` endpoint and the `ui-disabled` configuration, then verify that the API documentation is inaccessible when the UI is disabled.

Written by the indexing model from the issue text.