Document the new log and log.level playbook keys

The new playbook key log can be applied to the runtime key (see #145 (closed) and !652 (merged) for implementation notes). It accepts a list of optional built-in keys, including level. Documenting these two new keys and their capabilities includes the following tasks:

• Add a section about the log key to the playbook:configure-runtime.adoc page.
• Create the page with the file name runtime-log-level.adoc in the playbook module and add an xref to it in the module's navigation file.
• In the new page, describe the purpose of the level key, its default value, and its accepted values and their definitions in the new file.
• Show an example of the level key set in a playbook.
• Show an example of the --log-level option used in the CLI.

Additional updates and tasks include:

• Add the log.level, log.format, and log.failure_level keys to the available runtime keys reference table on the configure-runtime.adoc page
• Add the log keys to the CLI options table on the cli:options.adoc page.
• Add the log keys to the environment variables table on the playbook:environment-variables.adoc page

Note: The other log keys, log.format and log.failure_level will be handled by other issues/MRs.