Update webapp developer environment documentation

Update the documentation on the webapp developer environment to reflect boilerplate changes if uis/devops/webapp-boilerplate!72 (merged) is merged.

We still have a fair chunk of "old" boilerplate apps knocking around and so keep the existing documentation on a separate page.

We've been using https://gitlab.developers.cam.ac.uk/uis/devops/iam/identity/identity-system/#running-the-application as interim documentation. Changes which should be made:

• Include bullet-proof tutorial of how to initialise a new boilerplate webapp and get started with the dev environment.
• Make pipx install a suggestion but also allow local installation of poetry if the reader is comfortable with that.
• Include a brief overview of what poetry, poe and tox all do.
• Don't recommend pipx inject, use poetry self add which works even if poetry is installed locally.
• Note that editors like VS Code may need configuring to find poetry venvs: https://gitlab.developers.cam.ac.uk/uis/devops/gsuite/preferences-webapp/-/merge_requests/23#note_511701