ESQL: Architecture docs

[nik9000]

Description

We'd like to make some high level introductory docs for ESQL's architecture. It's got to be fairly simple so it doesn't go stale immediately but in depth enough to give folks a sense of the major components. Defines stuff like parser, logical plan, analyzer, physical plan, Attribute, EsField, compute engine, Operator, Block, Vector, ExpressionEvaluator, Aggregator, GroupingAggregator, Exchange. Maybe simple enough to make sure these have good javadoc, link to them, and

[elasticsearchmachine]

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

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Pinging @elastic/es-analytical-engine (Team:Analytics)

[nik9000]

We should also list out all of our magic code generation annotations. And probably link them in the appropriate spots.