Update Deployer guide

[ameotoko]

Small updates to the article about using Deployer.

• There's no need to reference an RC version, since Deployer 7 has been released (which also contains a bug fix to Contao recipe).

• An example on excluding folders from pre-set shared_dirs has been added. Not everybody uses Contao Manager and parameters.yml, but Contao recipe deploys them by default as empty directories.

• I find the wording about Contao "slowly migrating towards using public/ instead of web/" a little bit unfortunate. The guide claims to be intended for Contao 4.13 and up, yet public/ is already a default since 4.12