Add some documentation for how to add / update grid fields from an add-on

[jcogs-design]

Suggested Change Add some documentation / examples relating to how you might add / update a grid field from an add-on (outside of CP).

Reasoning Documentation is currently mute on this.

Additional context Seems odd that the grid field is relegated to the depths of legacy code. It appears that a grid field can be created by "making" a new instance of Channel Field of type 'grid' - but how do you access / add columns to the definition? I note that Datagrab (for example) interacts with Grid fields seemingly entirely via direct db access.