Update docs and wikis.

• Minimize amount of data presented at front page (README.md);
• Simplify examples presented at front page (README.md);
• Split wikis (docs) by functionality (a.k.a. API), not tasks (Howtos);
• Differentiate feature, functionality (API), and tasks (Howtos) docs;

Articles:

• Security
• Publish / Subscribe
• Using files without references and pub

In the following lines, we check if the visibility has been specified, and if it hasn't we default to private. https://github.com/Superbalist/flysystem-google-cloud-storage/blob/master/src/GoogleStorageAdapter.php#L143-L149

Is there any reason we need to default to anything at all? If we specify nothing, the default object ACL of the bucket should be applied (by default private, visible to owne