add global-config.adoc with asciidoc attributes that should be the same for all specs

[kbroch-rivosinc]

Rather than having to updated these across all spec repos when changes occur, a certain set should be consistent across the docs:

global-config.adoc:

:company: RISC-V
:url-riscv: http://riscv.org

Than that could be included in the template header.adoc file:

include::global-config.adoc[]

If this seems reasonable I can come up with a PR with a conservative set of attributes to add, and then put the include in the template.