doc: develop: Add 'Language Support' sub-category

This commit adds the 'Language Support' sub-category under the
'Developing with Zephyr' category with programming language support-
related documentations.

The contents of the 'C standard library' page have been relocated to
the 'C Language Support' page, and the contents of the 'C++ Support for
Applications' page have been relocated to the 'C++ Language Support'
page.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>