Docs: Improve ElasticSearch Documentation

We have seen areas where the documentation for ElasticSearch can be improved. Some general areas are:

• Async indexing spins up many Sidekiq processes (reference to Extra Sidekiq Processes)
• Information on tweaking max payloads
• Minimum specs for an ElasticSearch server (link to ElasticSearch Hardware Docs)
• Information on the rake tasks (and links to the code)
• Information on batch sizing
• More on troubleshooting