ARROW-11930 added a markdown file with some basic descriptions of ExecNode and ExecPlan. It'd be better if we made that into a full formal Sphinx document alongside compute.rst

Reporter: Ben Kietzman / @bkietz
Assignee: Ben Kietzman / @bkietz

Related issues:

• [C++] Query engine umbrella issue (is a child of)

PRs and other links:

• GitHub Pull Request #11309

_{Note: This issue was originally created as ARROW-13227. Please see the migration documentation for further details.}