Add section about Executor Builder App to Extension installation guide

[koettert]

With BH 1.10 the executor builder app is available which makes it easier to build an executor with the geospatial extension.

[sturmkev]

I have updated the documentation to include Automatic Image Creation (via the Data App) and Manual Image Creation.
Please have a look and comment if necessary.
geospatial-install-guide.zip

[koettert]

@koettert to revise the changes and commit them to the Github repo

[koettert]

Uploaded your changes to this branch @sturmkev https://github.com/spatial-data-lab/knime-geospatial-extension/blob/343-add-section-about-executor-builder-app-to-extension-installation-guide/docs/geospatial-install.md
If you need to do further editing you can do it directly on the branch.