Update documentation on tasks

[domoscargin]

What

Update docs/contributing/tasks.md and docs/contributing/coding-standards to remove references to gulp, and bring the descriptions of tasks up to date.

Why

We have made the decision to move away from Gulp and write our own build scripts.

Some of our guidance is already slightly out of sync with our current tasks, and will get more out of sync as we remove gulp from our tooling.

(There's a separate issue of how we prevent this guidance from getting out of sync in the future...)

Who needs to work on this

Developer, Technical Writer

Who needs to review this

Developer, Technical Writer

Done when

• [ ] We've decided which script aliases and tasks we're going to provide guidance for
• [ ] We've updated docs/contributing/tasks.md
• [ ] We've removed any references to gulp in the documentation