chore: inform triagebot of the contribution guidelines

[rami3l]

As suggested by @Kobzol, this PR tries to make use of the new triagebot features: https://forge.rust-lang.org/triagebot/pr-assignment.html#additional-new-pr-trigger-options

I have decided to remove the pull request template accordingly since it seems to be conveying the same information as the above options here.

[rami3l]

The part of the welcome message regarding the new config introduced here should look like so:

/// Add contribution instructions and/or LLM policy URLs to the given `message`, if configured.
pub fn add_contribution_urls(
    message: &mut String,
    contributing_url: Option<&str>,
    llm_policy: Option<&str>,
) {
    let msg = match (contributing_url, llm_policy) {
        (Some(contrib), Some(llm)) => Some(format!(
            "Please see [the contribution instructions]({contrib}) and our [LLM policy]({llm}) for more information."

https://github.com/rust-lang/triagebot/blob/f1b47efc88b8d9003612b1047f9c70b6269a2cc6/src/handlers/assign/messages.rs#L36-L44

[Kobzol]

Btw I think that keeping the PR template is still good, after all having this information before submitting a PR is better than only having it after the PR is opened.

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