Add "Rendered link" to PR to quick see the rendered changes

[Urgau]

This PR adds "Rendered link" to PR to quick see the rendered changes.

I have configured it to only trigger on files under the src/ directory.

Example

Note, despite the triagebot.toml being new, triagebot/rustbot is already activated for this repo.

[lqd]

We can only test this after merging, right?

[Urgau]

Test the effect yes, only after merging and having a new (or updated) PR.
The config is already validated, as triagebot hasn't complained.

You can however look at PRs in rust-lang/rfcs and blog.rust-lang.org which have the rendered links.

[lqd]

Let’s see then. Thanks!