Use custom iree.dev domain in links to documentation site. (#15036)

Follow-up to https://github.com/openxla/iree/pull/15008 and some GitHub
configuration changes that switched us to using a custom domain for our
GitHub pages hosting.

Background:

* We acquired this domain for hosting https://perf.iree.dev/ years ago,
with some aspirations to use the domain for other pages eventually.
* We tried to use the domain with our GitHub Pages content at one point,
but ran into some issues that resulted in buggy redirects and
certificate errors. A few weeks ago we tried that again and succeeded.

Note that we had also considered using the `docs` subdomain, i.e.
`https://docs.iree.dev/`. That still seems tempting, but would require
more domain configuration work. Until we use a subdomain like that or
explore the workarounds in
https://github.com/orgs/community/discussions/23184, the `www` part of
URLs is required.
diff --git a/README.md b/README.md
index 0d16563..065253c 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@
 that scales up to meet the needs of the datacenter and down to satisfy the
 constraints and special considerations of mobile and edge deployments.
 
-See [our website](https://openxla.github.io/iree/) for project details, user
+See [our website](https://iree.dev/) for project details, user
 guides, and instructions on building from source.
 
 [![CI Status](https://github.com/openxla/iree/actions/workflows/ci.yml/badge.svg?query=branch%3Amain+event%3Apush)](https://github.com/openxla/iree/actions/workflows/ci.yml?query=branch%3Amain+event%3Apush)
@@ -41,7 +41,7 @@
 ![IREE Architecture](docs/website/docs/assets/images/iree_architecture_dark.svg#gh-dark-mode-only)
 ![IREE Architecture](docs/website/docs/assets/images/iree_architecture.svg#gh-light-mode-only)
 
-See [our website](https://openxla.github.io/iree/) for more information.
+See [our website](https://iree.dev/) for more information.
 
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