Generate doc pages for iree-dialects. (#14388)

Progress on https://github.com/openxla/iree/issues/5477

This adds the `IREEInput` / `iree_input` and `IREELinalgExt` /
`iree_linalg_ext` dialects to
https://openxla.github.io/iree/reference/mlir-dialects/.

Preview: https://scotttodd.github.io/iree/reference/mlir-dialects/

* Added a new `iree-dialects-doc` CMake target alongside `iree-doc` and
`mlir-doc` (similar behavior, but just for
`llvm-external-projects/iree-dialects/`)
* Wired up `docs/website/generate_extra_files.sh` to build the new
target and plumb the files it generates through to the website folder
* Split "MLIR dialects" subsection of the website into "Internal
dialects" and "Public dialects"
* Added `OpDocGroup`s for the dialects and tweaked some comments in the
files

Note: I'm omitting `StructuredTransformOpsExt` and `LinalgTransformOps`
for now while the discussion at
https://github.com/openxla/iree/issues/5477#issuecomment-1626375703
resolves.

docs/website/mkdocs.yml

diff --git a/docs/website/mkdocs.yml b/docs/website/mkdocs.yml
index 7133ccb..64affe2 100644
--- a/docs/website/mkdocs.yml
+++ b/docs/website/mkdocs.yml
@@ -25,12 +25,12 @@
     - navigation.instant  # Ajax-style dynamic loading (faster)
     - navigation.tracking # Update URL in address bar with the active anchor
     - navigation.sections # Group sections without collapsible buttons
+    - navigation.expand   # Start sections expanded
     - navigation.tabs     # Show primary sections in tabs below the header
     - navigation.top      # "Back to top" button
     - navigation.indexes  # Section names can link to index.md pages
 
     # Disabled options (consider enabling as the site changes)
-    # - navigation.expand      # When 'sections' is disabled, start expanded
     # - navigation.tabs.sticky # Keep tabs visible at the top when scrolled
 
     - toc.follow # Scroll the TOC panel to follow the reader
@@ -145,15 +145,20 @@
       - "MLIR dialects":
           - "reference/mlir-dialects/index.md"
           # Note: these files are generated by generate_extra_files.sh.
-          - Check: "reference/mlir-dialects/Check.md"
-          - Flow: "reference/mlir-dialects/Flow.md"
-          - HAL: "reference/mlir-dialects/HAL.md"
-          - HALInline: "reference/mlir-dialects/HALInline.md"
-          - HALLoader: "reference/mlir-dialects/HALLoader.md"
-          - Stream: "reference/mlir-dialects/Stream.md"
-          - Util: "reference/mlir-dialects/Util.md"
-          - VM: "reference/mlir-dialects/VM.md"
-          - VMVX: "reference/mlir-dialects/VMVX.md"
+          - "Internal dialects":
+              - Check: "reference/mlir-dialects/Check.md"
+              - Flow: "reference/mlir-dialects/Flow.md"
+              - HAL: "reference/mlir-dialects/HAL.md"
+              - HALInline: "reference/mlir-dialects/HALInline.md"
+              - HALLoader: "reference/mlir-dialects/HALLoader.md"
+              - Stream: "reference/mlir-dialects/Stream.md"
+              - Util: "reference/mlir-dialects/Util.md"
+              - VM: "reference/mlir-dialects/VM.md"
+              - VMVX: "reference/mlir-dialects/VMVX.md"
+          # Dialects from llvm-external-projects/iree-dialects/
+          - "Public dialects":
+              - IREEInput: "reference/mlir-dialects/IREEInput.md"
+              - IREELinalgExt: "reference/mlir-dialects/IREELinalgExt.md"
       - "Other topics":
           - Glossary: "reference/glossary.md"
           - Optimization options: "reference/optimization-options.md"