doc/libgweather-sections.txt: Add missing symbols to the documentation
docs: Add 'Since:' tag in new API
doc/libgweather-docs.sgml: Add section to show new api added in 3.12
Change doc/libgweather-docs.sgml to doc/libgweather-docs.xml

Its a xml file now
doc/libgweather-docs.xml: Use the modern way to import XInclude declaration
docs: Add version information
docs: Document deprecated API