Documentation: cxd56xx and Spresense pages

[raiden00pl]

Since Sony is active in the development of NuttX (probably the only chip vendor here?), it would be nice if the Sony team would complete the documentation regarding cxd56xx and Spresense.

At the moment it is ugly and probably very incomplete:
https://nuttx.apache.org/docs/latest/platforms/arm/cxd56xx/index.html

ping @SPRESENSE

[acassis]

@jerpelea @masayuki2009 maybe some documentation from Spresense could be shared from yours website.