[Feature Request] Add custom commands before sphinx-build

[dgarcia360]

As a possible enhancement, the extension could provide the option to change the command used to build the docs.
Instead of using sphinx-build, I'd prefer to run any command such as build.sh so that I can run additional actions before and after each version is built.

Another option would be adding a --pre-build option to define a list of commands to run before the sphinx-build command is executed like in sphinx-autobuild

Please, let me know if you think this feature fits the project scope. If so, I'm willing to work on it and submit a PR. Thanks!