[Demo] Aliases are bleeding out of the demo section

[chalin]

This is a followup to:

• Reorganize demo feature documentation #3714

In particular because some of the configured aliases are bleeding out of the demo section, which we don't want. For example:

• https://opentelemetry.io/docs/feature_flags redirects into the Demo section at https://opentelemetry.io/docs/demo/feature-flags/. IMHO, we don't want this.

The redirect mentioned above is due to #3714 (comment).

cc @flands @cartermp

[chalin]

Oh, it's not as bad as I thought for the bleeding: only one page is bleeding out of the demo section. But the other aliases that were added are still wrong. I'll fix those too.