docs: make docs more consistent and complete (#9620)

- Add documentation for specgen along with proper math rendering
- Adjust the heading size, capitalization of various READMEs.
- Add backend test README to the doc
- Rename the CI related tests to have prefix "CI:"

docs_src/src_mdbook/src/SUMMARY.md

@@ -30,8 +30,9 @@
     - [Performance analysis](./notes/performance_analysis.md)
   - [Framegraph](./notes/framegraph.md)
   - [Tests](./notes/tests.md)
-    - [backend](./dup/test_backend.md)
-    - [renderdiff](./dup/test_renderdiff.md)
+    - [backend](./dup/backend_test.md)
+    - [CI: backend](./dup/test_ci_backend.md)
+    - [CI: renderdiff](./dup/test_ci_renderdiff.md)
   - [Libraries](./notes/libs.md)
     - [bluegl](./dup/bluegl.md)
     - [bluevk](./dup/bluevk.md)
@@ -46,9 +47,10 @@
     - [cmgen](./dup/cmgen.md)
     - [cso-lut](./dup/cso_lut.md)
     - [filamesh](./dup/filamesh.md)
-    - [normal-blending](./dup/normal_blending.md)
-    - [mipgen](./dup/mipgen.md)
     - [matinfo](./dup/matinfo.md)
+    - [mipgen](./dup/mipgen.md)
+    - [normal-blending](./dup/normal_blending.md)
     - [roughness-prefilter](./dup/roughness_prefilter.md)
+    - [specgen](./dup/specgen.md)
     - [specular-color](./dup/specular_color.md)
     - [zbloat](./dup/zbloat.md)