docs: Register i2c docs

I2C docs were added to the repository, but not registered in the doc
tree.

Change-Id: Ie4013d6febd6e379c0197906af086ee7d8543ba1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/45081
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
diff --git a/docs/BUILD.gn b/docs/BUILD.gn
index 26004b7..b01e53f 100644
--- a/docs/BUILD.gn
+++ b/docs/BUILD.gn
@@ -83,6 +83,7 @@
     "$dir_pw_fuzzer:docs",
     "$dir_pw_hdlc:docs",
     "$dir_pw_hex_dump:docs",
+    "$dir_pw_i2c:docs",
     "$dir_pw_interrupt:docs",
     "$dir_pw_interrupt_cortex_m:docs",
     "$dir_pw_kvs:docs",