Extend shell_command tutorial/how-to documentation with discussion of adhoc_tool

[chrisjrn]

Currently, the documentation lives in the "shell" section under "run shell commands".

This should live in a section on "extending Pants" (title tbd) that discusses use cases for adhoc_tool and shell_command, and discusses when one might want to write a plugin instead.

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Per #18082

PROPOSAL: adhoc_tool is documented as the preferred way to run third-party tools or user-supplied code as a build step, whenever it is possible. shell_command should be introduced only for cases where Pants does not provide any support.