* Moves `Discovery and cluster formation` content from `Modules` to
`Set up Elasticsearch`.
* Combines `Adding and removing nodes` with `Adding nodes to your
cluster`. Adds related redirect.
* Removes and redirects the `Modules` page.
* Rewrites parts of `Discovery and cluster formation` to remove `module`
references and meta references to the section.
This commit overhauls the documentation of discovery and cluster coordination,
removing mention of the Zen Discovery module and replacing it with docs for the
new cluster coordination mechanism introduced in 7.0.
Relates #32006
* Clean up s/ElasticSearch/Elasticsearch on docs/*
* Clean up s/ElasticSearch/Elasticsearch on src/* bin/* & pom.xml
* Clean up s/ElasticSearch/Elasticsearch on NOTICE.txt and README.textile
Closes#4634
James Rodewig
Lisa Cawley
David Turner
David Pilato
Simon Willnauer
Clinton Gormley