docs: Make labels consistent; remove boilerplate

- Update labels so they use consistent naming. Instead of the
  copy-and-pasted `chapter` prefix, name labels with a prefix consistent
  with what they point to:
  - "module-" for module
  - "target-" for targets
  - "docs-" for general documentation
- Remove the unnecessary highlight and default-domain directives.
  Neither is necessary since the Pigweed docs always specify the
  language in code snippets and other directives.

Change-Id: I55c34e9ec919f6f4670e5d3d008e0edf5fecf05b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/19328
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/pw_span/docs.rst b/pw_span/docs.rst
index 64dad43..2987ce4 100644
--- a/pw_span/docs.rst
+++ b/pw_span/docs.rst
@@ -1,8 +1,4 @@
-.. _chapter-pw-span:
-
-.. default-domain:: cpp
-
-.. highlight:: sh
+.. _module-pw_span:
 
 -------
 pw_span