feat(root): added ag-grid theme guidance

[GCHQ-Developer-741]

Summary of the changes

Adding ag-grid guidance to v3 branch

Checklist

• [x] I have manually accessibility tested any changes, if relevant.
• [x] I have raised this pull request against the develop branch

[github-actions[bot]]

View your branch deployment here: https://mi6.github.io/ic-design-system-githubpages/branches/ag-grid-guidance-on-v3