Improve documentation templates (#618)

- Adds CI to check that the documentation templates are up-to-date.
- Add a new `help` command to the documentation templates. This command
  prints the help message for all the sub-commands of `pgcopydb` in a
  single file. This is useful for users who want to see all the
  sub-commands in one place.
- Remove dependency on installing the binary before updating the
  documentation templates. This allows users to update the documentation
  templates without installing the binary. One can set the `PGCOPYDB`
  environment variable to point to the binary to use for updating the
  documentation templates.
- Fix a casing issue in command descriptions. The first letter of the
  first word in the description of each command should be capitalized.