doc: move namespaces.rst from kbuild/ to core-api/

We discussed a better location for this file, and agreed that
core-api/ is a good fit. Rename it to symbol-namespaces.rst
for disambiguation, and also add it to index.rst and MAINTAINERS.

Signed-off-by: Masahiro Yamada <[e-mail]>
Acked-by: Matthias Maennich <[e-mail]>
Signed-off-by: Jessica Yu <[e-mail]>

Documentation/core-api/index.rst

···
    protection-keys
    ../RCU/index
    gcc-plugins
+   symbol-namespaces
 
 
 Interfaces for kernel debugging

MAINTAINERS

···
 M:	Matthias Maennich <[e-mail]>
 S:	Maintained
 F:	scripts/nsdeps
+F:	Documentation/core-api/symbol-namespaces.rst
 
 NTB AMD DRIVER
 M:	Shyam Sundar S K <[e-mail]>