Update internal-ui-mounts.mdx

[stuartpurgavie]

For sys/internal/ui/mounts: Make it clear that this endpoint is not only being leveraged on the Web UI, but is also leveraged in other places such as command/kv_helpers.go for example. Also update response to include options data present on KV v2 mounts.

For sys/internal/ui/mounts/:path: Add initial version of this endpoint family.

[stuartpurgavie]

Cannot find a changelog example of a documentation-only change to satisfy the "Check Changelog" PR check.

[taoism4504]

@hsimon-hashicorp , hi. This needs to be technically reviewed. Thanks!

[heatherezell]

Thanks for your contribution @stuartpurgavie! :)