Refine the fileset which requires copyright disclaimer

[yuri-sergiichuk]

While working on #155 we've realized that we don't really want to add the disclaimer to all files in repositories. E.g. there's no need to add the disclaimer to .gitignore or .gitattributes or other config-only files.

The scope of this issue is:

1. Define which files must and which must not have the disclaimer.
2. Update the config module by removing/adding the disclaimer to appropriate files.
3. Consider updating other repos using defined rules.