pw_thread: add helper ThreadCore interface

Also extends the pw_thread documentation regarding thread creation.

Change-Id: Ib0a0572060188244b9a5cf91a6bba575057f405b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/41205
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Reviewed-by: Paul Mathieu <paulmathieu@google.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pw_thread/BUILD b/pw_thread/BUILD
index fb892c0..71957c0 100644
--- a/pw_thread/BUILD
+++ b/pw_thread/BUILD
@@ -101,8 +101,12 @@
     name = "thread",
     deps = [
         ":thread_facade",
+        ":thread_core",
         PW_THREAD_THREAD_BACKEND + "_headers",
     ],
+    srcs = [
+        "thread.cc"
+    ],
 )
 
 pw_cc_library(
@@ -113,6 +117,14 @@
 )
 
 pw_cc_library(
+    name = "thread_core",
+    hdrs = [
+        "public/pw_thread/thread_core.h",
+    ],
+    includes = ["public"],
+)
+
+pw_cc_library(
     name = "yield_facade",
     hdrs = [
         "public/pw_thread/yield.h",