[DOCS] Document anatomy of a search request

[jrodewig]

We should consider adding an "anatomy of a search request" section that walks users through the various phases of search execution. While not required to run a search, understanding these phases can be helpful when troubleshooting issues. It can also help users better understand recent improvements made to search execution.

Relates to:

• https://github.com/elastic/elasticsearch/pull/78765
• https://github.com/elastic/elasticsearch/pull/79212

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[lockewritesdocs]

This is a great idea, and I think that an infographic could be very useful to help explain the different phases.

[jrodewig]

Related blog post: https://www.elastic.co/blog/three-ways-improved-elasticsearch-scalability/