Move foyer package from gh-pages to r-universe.

Distribute foyer package via r-universe. This
removes the need to create a custom cran-like
repository hosted on github pages. r-universe
provides this functionality by linking to a
github repository that points to the foyer package
(https://github.com/SimpleITK/simpleitk.r-universe.dev).

Author: zivy

.github/workflows/main.yml

@@ -177,15 +177,15 @@ jobs:
 
           Detailed release notes are available on the [main SimpleITK repository](https://github.com/SimpleITK/SimpleITK/releases/${{ github.ref_name }}).
 
-          To install SimpleITK we use a Foyer helper package that downloads the appropriate binary from the GitHub release assets. This is a two step process.
+          To install SimpleITK we use a Foyer helper package that downloads the appropriate binary. This is a two step process.
 
           To install the latest SimpleITK version to your primary library directory, first element of `.libPaths()`, run the following:
 
           ```r
           # install the SimpleITK foyer package
           install.packages(
             "SimpleITK.foyer",
-            repos = c("https://${{ github.repository_owner }}.github.io/${{ github.event.repository.name }}/"),
+            repos = c("https://simpleitk.r-universe.dev"),
             type = "source"
           )
 
@@ -207,6 +207,7 @@ jobs:
 
           If you directly download the package artifact from this release page, before you install the package you will need to first unzip the file. Then rename it to `SimpleITK_<version>.zip` (windows), `SimpleITK_<version>.tgz` (macOS), or `SimpleITK_<version>.tar.gz` (Linux) to match the expected file name format.
 
+          If you need a custom build of SimpleITK or the binary package for your platform, R version, or desired SimpleITK version is not available you will need to build the package yourself. To do this, use the [remotes based installer](https://github.com/SimpleITK/SimpleITKRInstaller).
         files: |
           release-artifacts/*
         fail_on_unmatched_files: true

.github/workflows/update_gh_cran.yml

@@ -0,0 +1,29 @@
+# Release Instructions
+
+This document describes the steps required to create the binary R release for SimpleITK which updates and relies on the remotes based SimpleITKRInstaller.
+
+1. Create branch off of `main` and switch to it,  `git checkout -b updateRelease main`.
+
+1. Update the installer `DESCRIPTION` file: change the `SITK_TARGET` field to the desired SimpleITK version (e.g. v2.5.5) and run the version update script (in bash or Git Bash on windows), `sitk_r_version_date.sh`. This script will update the file's `Version` and `Date` fields according to the `SITK_TARGET`.
+
+1. Commit and push changes:
+  
+    ```
+    git commit -am "Update SimpleITK release"
+    git push origin updateRelease
+    ```
+
+1. On GitHub create a PR, see that all tests pass and binary artifacts are created successfuly. Merge into main.
+
+1. Update local `main` from the remote and check out the `main` branch. Tag it using the **exact tag** you listed in the `SITK_TARGET` and push the tag to this repository.
+
+   ```
+   git tag v2.5.5
+   git push https://github.com/SimpleITK/SimpleITKRInstaller.git v2.5.5
+   ```
+
+1. Monitor the build process, https://github.com/SimpleITK/SimpleITKRInstaller/actions. If a build fails due to transient issues, rerun from failed (binary artifacts created for successful builds are retained across reruns). 
+
+1. Once all builds complete successfully a [draft release](https://github.com/SimpleITK/SimpleITKRInstaller/releases) is automatically created. Verify that the expected binary packages are available as release artifacts and test them (download, unzip, rename and install).
+
+1. Edit and publish the release. Check that the binary distribution is working as expected via the r-universe distribution of the foyer package (follow the usage instructions provided in the GitHub release notes).