Add markdown link to diagnostic

[TheBlckbird]

Please write a short comment explaining your change (or "none" for internal only changes)

changelog: changes the link in diagnostics to a markdown link so editors like VSCode can create a clickable link

[rustbot]

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[TheBlckbird]

Whoops, I just realized that I forgot something important. I’ll fix it later today.

[flip1995]

This doesn't work, as the command line output would now contain the lint twice. As our main focus of error messages and suggestions is the CLI, I can't accept this PR. VSCode and other editors should be able to parse links without relying on markdown formatting IMHO.

error: approximate value of `f{32, 64}::consts::PI` found
 --> main.rs:1:17
  |
1 | const PI: f32 = 3.14159;
  |                 ^^^^^^^
  |
  = help: consider using the constant directly
  = help: for further information visit [rust-lang.github.io/rust-clippy/master/index.html#approx_constant](https://rust-lang.github.io/rust-clippy/master/index.html#approx_constant)
  = note: `#[deny(clippy::approx_constant)]` on by default

Thanks for trying to improve Clippy!

[TheBlckbird]

I think I‘ll open an issue in the VSCode repo

[TheBlckbird]

microsoft/vscode#206469