Copyedit scripting & python sections

[Tom-Willemsen]

General edit & tidy-up of scripting & python sections.

Most content was not too bad/out-of-date, so the changes are relatively minor:

• Cross-link to various other documentation more thoroughly, using sphinx refs where possible to ensure links are checked and don't become stale
• Move uv notes to backend: they are relevant to the backend setup, not to user-facing scripting workflows
• Expand on uv venvs in a few places