fix: add anchor-spl/idl-build in idl-build feature list in documentation

[hthuz]

I'm trying to follow the doc to create a token mint locally. However, I met the following compilation error:

error[E0277]: the trait bound `anchor_spl::token_interface::Mint: Discriminator` is not satisfied
  --> programs/sol-example/src/lib.rs:29:39
   |
29 |     pub mint: InterfaceAccount<'info, Mint>,
   |                                       ^^^^ the trait `Discriminator` is not implemented for `anchor_spl::token_interface::Mint`

from the struct

#[derive(Accounts)]
pub struct CreateMint<'info> {
    #[account(mut)]
    pub signer: Signer<'info>,
    #[account(
        init,
        payer = signer,
        mint::decimals = 6,
        mint::authority = signer.key(),
        mint::freeze_authority = signer.key(),
    )]

    pub mint: InterfaceAccount<'info, Mint>,
    pub token_program: Interface<'info, TokenInterface>,
    pub system_program: Program<'info, System>,
}

I tracked #3151 and #2745 and found that the documentation doesn't mention the anchor-spl/idl-build feature, which causes the compilation error. This PR adds this requirement to the documentation.

[vercel]

@hthuz is attempting to deploy a commit to the coral-xyz Team on Vercel.

A member of the Team first needs to authorize it.