This patch adds the `./mach perfdocs` command to mozperftest. It also sets up the logger required by perfdocs for producing reviewbot-compatible lint errors, and output these into a JSON file. A `--generate` option is added to enable re-generation of the documentation. At the same time, the existing lint task is changed to use this new command. Because of a slight change in how perfdocs works, this patch also fixes a bug with the talos gatherer that had caused an existing failure to slip through. Differential Revision: https://phabricator.services.mozilla.com/D297130