tools: Add docs subcommand docs which opens browser with documentation for current version of idf and model of your target

Add docs subcommand with options:
--no-browser - Prints url for current documentation instead of opens browser with documentation
--language - Select documentation language
--starting-page - Choice section of documentation to open
--version - Choice version of esp-idf
--target - Choice model of your target

docs/en/api-guides/build-system.rst

@@ -102,6 +102,7 @@ Advanced Commands
 - ``idf.py size`` prints some size information about the app. ``size-components`` and ``size-files`` are similar commands which print more detailed per-component or per-source-file information, respectively. If you define variable ``-DOUTPUT_JSON=1`` when running CMake (or ``idf.py``), the output will be formatted as JSON not as human readable text. See ``idf.py-size`` for more information.
 - ``idf.py reconfigure`` re-runs CMake_ even if it doesn't seem to need re-running. This isn't necessary during normal usage, but can be useful after adding/removing files from the source tree, or when modifying CMake cache variables. For example, ``idf.py -DNAME='VALUE' reconfigure`` can be used to set variable ``NAME`` in CMake cache to value ``VALUE``.
 - ``idf.py python-clean`` deletes generated Python byte code from the IDF directory which may cause issues when switching between IDF and Python versions. It is advised to run this target after switching versions of Python.
+- ``idf.py docs`` will open direct link to documentation for project's chip target and version in browser. To see all options use ``idf.py docs --help``
 
 The order of multiple ``idf.py`` commands on the same invocation is not important, they will automatically be executed in the correct order for everything to take effect (ie building before flashing, erasing before flashing, etc.).