Documentation: review supported POSIX interfaces in NuttX

Add a page in the documentation listing supported and unsupported POSIX options.
This is the first step to support POSIX Application Environment Profiles.

Signed-off-by: p-szafonimateusz <p-szafonimateusz@xiaomi.com>

Documentation/index.rst

@@ -38,6 +38,7 @@ Last Updated: |today|
    faq/index.rst
    debugging/index.rst
    guides/index.rst
+   standards/index.rst
    glossary.rst
    logos/index.rst
    _tags/tagsindex.rst

Documentation/standards/index.rst

@@ -0,0 +1,8 @@
+=========
+Standards
+=========
+
+.. toctree::
+   :maxdepth: 2
+
+   posix.rst