Add ci, docs, examples to export ignore via gitattributes file

[alexander-schranz]

This make the installed packages smaller, this files are not required to run the library.

Reference: https://getcomposer.org/doc/02-libraries.md#light-weight-distribution-packages Example: https://github.com/Seldaek/monolog/blob/3.5.0/.gitattributes

[tschoffelen]

Released in v2.3.1