docs: address signals flywheel review feedback

Addresses review comments on #910:

- Shorten the paper citation to (Chen et al., 2026) per common cite
  practice (replacing the full author list form).
- Replace the Why Signals Matter section with the review-suggested
  rewrite verbatim: more formal intro framing, renumbered steps to
  Instrument / Sample & triage / Data Construction / Model Optimization
  / Deploy, removes 'routing decisions' from the data-construction
  step, and adds DPO/RLHF/SFT as model-optimization examples.
- Renders tau and O(messages) as proper math glyphs via the sphinx
  built-in :math: role (enabled by adding sphinx.ext.mathjax to
  conf.py). Using the RST role form rather than raw $...$ inline so
  sphinx only injects MathJax on pages that actually have math,
  instead of loading ~1MB of JS on every page.

Build verified locally: sphinx-build produces no warnings on the
changed files and the rendered HTML wraps tau and O(messages) in
MathJax-ready <span class="math">\(\tau\)</span> containers.

Made-with: Cursor

docs/source/conf.py

@@ -33,6 +33,7 @@ extensions = [
     "sphinx.ext.autodoc",
     "sphinx.ext.intersphinx",
     "sphinx.ext.extlinks",
+    "sphinx.ext.mathjax",
     "sphinx.ext.viewcode",
     "sphinx_sitemap",
     "sphinx_design",
@@ -41,6 +42,7 @@ extensions = [
     "provider_models",
 ]
 
+
 # Paths that contain templates, relative to this directory.
 templates_path = ["_templates"]