Create a compute page in the C++ section of the Sphinx docs and make a list of the available functions and what they do

Reporter: Wes McKinney / @wesm
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [C++] C++ array kernels framework and execution buildout (umbrella issue) (relates to)
• [C++] Document more of the compute layer (is depended upon by)

PRs and other links:

• GitHub Pull Request #7695

_{Note: This issue was originally created as ARROW-8989. Please see the migration documentation for further details.}