Feature/smart site update

[antony]

Whilst working on the various PRs, I noticed that the update scripts are easily overlooked and don't produce meaningful errors, requiring diving into the readme.

This PR:

• Names them consistently
• Does some smarter checking regarding the need to run them locally or not
• Always forces a run when building for production
• Runs scripts in parallel where possible
• Uses path.join rather than string concatenation where possible in the scripts.
• Simplifies the readme instructions

Before submitting the PR, please make sure you do the following

• This message body should clearly illustrate what problems it solves. If there are related issues, remember to reference them.

[antony]

Hm - I appear to have checked a cache dir in! Converted to draft whilst I fix.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.