Update developer docs and env file

[MridulS]

Update the developer env file https://scipp.github.io/development/getting-started.html which is recommened in the developer starter guide https://scipp.github.io/development/getting-started.html. Ideally it should be autoupdated from a single source of truth. The developer docs also recommends to use commands like conda develop which is (kind-of) deprecated. The recommended way is pip install --no-build-isolation --no-deps -e .

Just putting this as an issue, so I we don't lose track of this.