Generate and publish API reference pages

We have enough user-facing APIs to add a dedicated `Reference` section on our user website.

## Python bindings

Most of our Python classes and functions have docstrings, but the files are scattered throughout our codebase and split across several module namespaces and pip packages.

We could use `mkdocstrings` with our new mkdocs site:
* https://mkdocstrings.github.io/
* https://chrieke.medium.com/documenting-a-python-package-with-code-reference-via-mkdocs-material-b4a45197f95b

https://wiki.python.org/moin/DocumentationTools has more options. [Doxygen](https://www.doxygen.nl/index.html) could be nice, as it supports all the languages we use.

## TFLite bindings

TFLite / TFLite Java bindings can probably just link to TFLite's docs for API references, with differences highlighted on our pages.

## C API

The API headers are well documented (example: [`iree/base/api.h`](https://github.com/google/iree/blob/main/iree/base/api.h)) but the documentation is split across many files (example: [`iree/vm/api.h`](https://github.com/google/iree/blob/main/iree/vm/api.h) just exports various headers from `iree/vm/`). Aggregating into reference pages like https://mlir.llvm.org/docs/LangRef/ or https://mlir.llvm.org/doxygen/namespacemlir.html could be helpful.

## Java bindings

Our Java bindings are experimental and incomplete at the moment. If we develop them into a larger, supported API, we could use Javadoc or Doxygen.

## MLIR Dialects

We generate documentation for our MLIR dialects and publish them today at https://google.github.io/iree/Dialects. These are not user-facing (for most, but not all, definitions of "IREE user"), but it's useful to discuss them here too. Tablegen doc generation uses https://github.com/google/iree/blob/main/build_tools/cmake/iree_tablegen_doc.cmake to generate markdown files