arm64: Add tagged-address-abi.rst to index.rst

Documentation/arm64/tagged-address-abi.rst introduces the
relaxation of the syscall ABI that allows userspace to pass
certain tagged pointers to kernel syscalls.

Add the document to index.rst for a correct generation of the
table of content.

Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

Documentation/arm64/index.rst

@@ -16,6 +16,7 @@ ARM64 Architecture
     pointer-authentication
     silicon-errata
     sve
+    tagged-address-abi
     tagged-pointers
 
 .. only::  subproject and html