Revises the current 'How to avoid oversharding' docs to incorporate
information from our [shard sizing blog post][0].
Changes:
* Streamlines introduction
* Adds "Things to remember" section to describe how shards work
* Adds "Guidelines" section based on blog tips
* Creates a "Fix an oversharded cluster" section
[0]: https://www.elastic.co/blog/how-many-shards-should-i-have-in-my-elasticsearch-cluster
James Rodewig