Fix Sphinx nitpick warning, link check errors and spelling mistakes

[satterly]

See https://github.com/alerta/alerta-docs/runs/4318069695?check_suite_focus=true

[sixcare]

I've been able to remove most of the warnings: https://github.com/sixcare/alerta-docs/runs/4500242883?check_suite_focus=true

[satterly]

@sixcare awesome, thanks! please submit a PR.