docs: cleaned up and improved docs for writing tasks

Extracted example-task.rst from tasks.rst.

Extracted multihost.rst from tests.rst. Dropped remaining content from
tests.rst, because it was excessively preachy but not particularly
instructive or relevant to Beaker.

Converted makefile.rst to task-metadata.rst, and dropped references to
Makefile variables which are no longer relevant to Beaker.

Wrote task-environment.rst, describing commands and environment
variables for tasks.

Added writing-tasks.rst to group it all together in a new top-level
section.

Bug: 872421
Change-Id: I313017a7122729b2a9f387e47ed218c903dffe2b
Merge "docs: cleaned up and improved docs for writing tasks" into develop