Remove NaturalDocs as mandatory requirements

[Jeevhi]

cmake fails if NaturalDocs not found.

-- Could NOT find NaturalDocs (missing: NATURALDOCS_EXECUTABLE) (Required is at least version "1.51")

For most people, the online document is sufficient. Please consider removing it or making it optional

[vapier]

are you building a release or from git?

[Jeevhi]

git

[vapier]

it's not unreasonable to require extra tools when building from git. if you want simpler requirements, use a release.