Add legal header to files in docs/ directory

[borisrizov-zf]

According to TRG 7.07 ALL documentation should at the very least have a notice section included describing the license. The docs/ directory holds many files which are getting converted to a single doc.

We should add a NOTICE.md file and append it to the end of the generated documentation to make the generated doc compliant.

[borisrizov-zf]

After brief discussion in the Office Hour, we've agreed to add a single NOTICE.md which will include the legal information as per TRG 7.07
AND
add a notice in the main NOTICES.md to point out that all files in the docs/ directory are subject to the license.