[roll third_party/pigweed] docs: How to use a single Pigweed module in Bazel

Provide some guidance on how to depend on Pigweed as "just a library"
from a pre-existing Bazel project.

In response to an old request from Kayce (sorry, internal-only link:
https://docs.google.com/document/d/1HkccKsBmC1IALJMszrLi9LBnK9mpHaiytL66txozVJU/edit?resourcekey=0-pIsK8e1pXn_CGdX-IYRyTg&disco=AAAA_QKPhdM)

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/188922

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 853989651ead5f5..91075ebb2367073
Roller-URL: https://ci.chromium.org/b/8757993488216923329
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I0e6c9beec9a9230a518a06a73b62be94f6fd9f50
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/189094
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 8539896..91075eb 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 853989651ead5f5ae95f0698ca460be6e29b11b6
+Subproject commit 91075ebb2367073053be359a3baa17b9fa526b70