[PA] Link to other docs

This change puts links to additional docs in a dedicated section atop
the primary README.

Change-Id: If87da86fe32e5ab08967a21e6fcc8bd48c57b649
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3812318
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1032474}

base/allocator/partition_allocator/PartitionAlloc.md

@@ -4,16 +4,19 @@ This document describes PartitionAlloc at a high level, with some architectural
 details. For implementation details, see the comments in
 `partition_alloc_constants.h`.
 
+## Quick Links
+
+* [Glossary](./glossary.md): Definitions of terms commonly used in
+  PartitionAlloc. The present document largely avoids defining terms.
+
+* [Build Config](./build_config.md): Pertinent GN args, buildflags, and
+  macros.
+
 ## Overview
 
 PartitionAlloc is a memory allocator optimized for space efficiency,
 allocation latency, and security.
 
-*** note
-This document largely avoids defining terms; consult the
-[glossary](./glossary.md) for a complete reference.
-***
-
 ### Performance
 
 PartitionAlloc is designed to be extremely fast in its fast paths. The fast