docs(gpt): add documentation page for GPT library

This patch adds some documentation for the GPT library as well as adds
code owners for it.

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: If1cd79626eadb27e1024d731b26ee2e20af74a66
diff --git a/docs/components/index.rst b/docs/components/index.rst
index 754526d..95fe42c 100644
--- a/docs/components/index.rst
+++ b/docs/components/index.rst
@@ -25,3 +25,4 @@
    xlat-tables-lib-v2-design
    cot-binding
    realm-management-extension
+   granule-protection-tables-design