docs update : Installation copy-assets script

[shaielc]

Added the copy-thebe-assets script and referenced it in the relevant docs.

This resolves #645

If I am understanding correctly this stage is required whenever using either thebe-lite or thebe-core, so I put the script in thebe-core.

I have also made it optional to have thebe-lite installed but later I saw the it is a dependency of thebe-core which makes this change kinda moot

[shaielc]

I have tested it by running:

cd packages/core
npm run build
npm pack
mkdir /tmp/test
cd /tmp/test
npm init
npm install <path to rep>/packages/core/thebe-core-0.2.3.tgz
npx copy-thebe-assets thebe

[rowanc1]

This is great. I am using this over in another myst-theme as well, and it will be nice to not have to copy this script all over the place!

[shaielc]

The resource not accessible issue seems like a problen with the CI
I am guessing the cause is that this PR is from a fork

[stevejpurves]

@shaielc yes that's a limitation with the preview action.
Thanks for your PR!

[stevejpurves]

this will be released in v0.2.5 shortly, please disregard v0.2.4