Make the AI Policy more visible to newcomers (#23420)

# Objective

We adopted an AI Policy regarding LLM assisted/generated PRs, but due to
https://github.com/bevyengine/bevy/pull/23276 slipping through, it
became clear this wasn't being surfaced to newcomers more easily,
leading to some confusion.

So to alleviate concerns raised by
https://github.com/bevyengine/bevy/issues/23418, we should rectify that.

## Solution

Simple, just link the AI Policy directly to newcomers. Links added to
the welcoming message and the CONTRIBUTING.md should at least allow a
more immediate notice to new contributors about our stance on AI
usage/contributions.

Further work is updating the Introduction page on the website to also
link to the AI Policy, perhaps with a more summarised explanation. If
this policy is important, it needs to be aggressively linked to and
mentioned so that readers find their way to it more easily.

## Testing

How do we test the welcoming message function? 😄

.github/workflows/welcome.yml

@@ -45,5 +45,5 @@ jobs:
               repo: context.repo.repo,
               body: `**Welcome**, new contributor!
 
-              Please make sure you've read our [contributing guide](https://bevy.org/learn/contribute/introduction) and we look forward to reviewing your pull request shortly ✨`
+              Please make sure you've read our [contributing guide](https://bevy.org/learn/contribute/introduction), as well as our [policy regarding AI usage](https://bevy.org/learn/contribute/policies/ai/), and we look forward to reviewing your pull request shortly ✨`
             })

CONTRIBUTING.md

@@ -1,4 +1,4 @@
 # Contributing to Bevy
 
 If you'd like to help build Bevy, start by reading this
-[introduction](https://bevy.org/learn/contribute/introduction). Thanks for contributing!
+[introduction](https://bevy.org/learn/contribute/introduction), as well as our [policy regarding AI usage](https://bevy.org/learn/contribute/policies/ai/). Thanks for contributing!