Add CI dev doc for in-house GitHub actions

This doc describes how we can reuse our in-house actions as our CI continues to grow.

Signed-off-by: Courtney Pacheco <6019922+courtneypacheco@users.noreply.github.com>

Author: courtneypacheco

docs/ci/ci-universal-actions.md

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+# Create Separate Repo for Custom GitHub Actions
+
+## Idea Overview
+
+Create a separate repository within the `instructlab` GitHub org to house our custom CI Github Actions.
+
+## Motivation for this Proposal
+
+Within our `instructlab` GitHub org, we have an in-house GitHub action that we use across several repos: `free-disk-space`.
+
+Examples:
+  - [eval's `free-disk-space`](https://github.com/instructlab/eval/blob/main/.github/actions/free-disk-space/action.yml)
+  - [instructlab's `free-disk-space`](https://github.com/instructlab/instructlab/blob/main/.github/actions/free-disk-space/action.yml)
+  - [sdg's `free-disk-space`](https://github.com/instructlab/sdg/blob/a532a8d99ffe447152948e9442464923aba62637/.github/actions/free-disk-space/action.yml)
+
+This GitHub action is universal as it is a simple script used to clean up disk space on GitHub runners and can be modified to free disk space on our CentOS-based EC2 runners.
+
+Note that all of these in-house `free-disk-space` action files are exactly the same, so we essentially have the same file stored in three different repos.
+
+## Pros and Cons
+
+Below are some pros and cons of creating a separate repository to house our GitHub actions.
+
+### Pros
+
+  - We will have one action file in one location
+    - Easier to make changes in one location instead of (our present) three locations
+    - Contributors will know where to look for in-house action files
+  - We can create releases and utilize version control
+
+### Cons
+
+  - Extra repository to maintain.
+  - We can't publish any of our actions to the GitHub marketplace if we have multiple actions stored in one repository.\*
+
+\* In reference to the last bullet point, [the GitHub docs for publishing Actions](https://docs.github.com/en/actions/sharing-automations/creating-actions/publishing-actions-in-github-marketplace#about-publishing-actions) states:
+> Actions are published to GitHub Marketplace immediately and aren't reviewed by GitHub as long as they meet these requirements:
+> 
+> - The action must be in a public repository.
+> - Each repository must contain a single action.
+
+If we do care about publishing our actions, then we should consider creating separate repositories for these actions. If we don't care to publish, then this is a non-issue. (See next section below.)
+
+## Additional Info
+
+Even if we cannot publish our actions to the GitHub marketplace, we can _still_ use these actions in our repository. For example, if our repo was named `ci-actions` with this layout:
+
+```
+.
+├── custom-action-1/
+│   ├── action.yml
+├── custom-action-2
+│   ├── action.yml
+```
+
+...then we'd reference them in our other repos like so:
+
+```
+name: Some Name
+
+on:
+  workflow_dispatch:
+
+jobs:
+  some-job:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout
+  
+      - name: Use custom action 1
+        uses: instructlab/ci-actions/custom-action-1@main
+  
+      - name: Use custom action 2
+        uses: instructlab/ci-actions/custom-action-2@main
+```
+
+Reference: https://stackoverflow.com/a/79100136