feat: add //python:none as public target to disable exec_interpreter

[rickeylev]

When writing the toolchain docs, I realized there wasn't a public target to use for
disabling the exec_interpreter.

Fixed by adding an alias to the internal target.

Along the way:

• Add the exec tools and cc toolchains to the doc gen
• A few improvements to the cc/exec tools docs
• Add public bzl file for py_exec_tools_toolchain and PyExecToolsInfo
• Fix a bug in sphinx_bzl where local names were hiding global names even if the
  requested type didn't match (e.g. a macro foo referring to rule foo)
• Fix xrefs in the python/cc/index.md; it wasn't setting the default domain to bzl
• Fix object type definition for attributes: the object type name was "attribute",
  but everything else was using "attr"; switched to "attr"