Refactor import hierarchy and API documentation

• simplify, homogenize and make more explicit the definition of what is "public" API and what are internal implementations. For this we now declare the __all__ variable for (and only for) the public API modules. We also remove all usages of import * in the public API.
• refactor the api autodocumentation tool to make use of the explicit public API definition (fixes #1011 (closed))
• refactor the imports of taurus to avoid importing taurus.core or taurus.customsettings when importing taurus (fixes #1217 (closed))
• improve documentation templates:
  • document all public members of modules (not only classes and functions), but also variables and exceptions
  • add hint for suggested import style in classes
  • add small summary definition for classes in the modules (and a link to expand the info)

This MR is also relevant to simplify the resolution of #1067 (closed) and also of #1181 (closed)

Note that the new documentation is built and can be reviewed by clicking the ~View App` button.