Update platform support page to note Solaris x86-64 1.0.46 release.
While here, explicitly output pages as UTF-8 (we already output a meta
header indicating the pages were so encoded, but the actual encoding
depended on the environment in which the pages were built).
Also, add a note to README pointing out that the pages need to be built in
an 8-bit clean environment due to ISO-8859-1 chars in sbcl/NEWS -- although
the code specifies the charset to use when reading and writing files,
news.lisp-expr is loaded via ASDF, which does not provide a means (that I
found) to specify the :external-format of a source file.