[doc]: NVS documentation updates

* Move nvs flash README to common doc directory
* correct markup of functions and types in text
  from old README
* Better comment of nvs_get_used_entry_count()
* Mention C++ example in API reference
* Used target instead of hard code ESP32
* Note that strings can only span one page
* Reflect that item types have been moved
* Some clarification about nvs_commit()
* Improved reference to the ESP Partition API
* fixed little mistake in documenting-code.rst
* Change of nvs_open_from_part() to
  nvs_open_from_partition() reflected in docs
* Corrected documentation of
  NVSHandle::get_string(), NVSHandle::get_blob()
  and NVSHandle::get_item_size().

* Closes DOC-165
* Closes IDF-1563
* Closes IDF-859
* Closes https://github.com/espressif/esp-idf/issues/6123

docs/en/contribute/documenting-code.rst

@@ -376,7 +376,7 @@ Once documentation is ready, follow instruction in :doc:`../api-reference/templa
 OK, but I am new to Sphinx!
 ---------------------------
 
-1. No worries. All the software you need is well documented. It is also open source and free. Start by checking `Sphinx`_ documentation. If you are not clear how to write using rst markup language, see `reStructuredText Primer <https://www.sphinx-doc.org/en/stable/rest.html>`_. You can also use markdown (.md) files, and find out about more about the specific markdown syntax that we use on `Recommonmark parser's documentation page <https://recommonmark.readthedocs.io/en/latest/>`_.
+1. No worries. All the software you need is well documented. It is also open source and free. Start by checking `Sphinx`_ documentation. If you are not clear how to write using rst markup language, see `reStructuredText Primer <https://www.sphinx-doc.org/en/stable/rest.html>`_. You can also use markdown (.md) files, and find out more about the specific markdown syntax that we use on `Recommonmark parser's documentation page <https://recommonmark.readthedocs.io/en/latest/>`_.
 
 2. Check the source files of this documentation to understand what is behind of what you see now on the screen. Sources are maintained on GitHub in `espressif/esp-idf`_ repository in :idf:`docs` folder. You can go directly to the source file of this page by scrolling up and clicking the link in the top right corner. When on GitHub, see what's really inside, open source files by clicking ``Raw`` button.