migrate ReadTheDocs to new app

[orbeckst]

• enable org wide
• delete old webhooks
• enable repo in RTD settings (connect)
• enable "Show build overview in comment"

[orbeckst]

In principle the migration is complete; RTD does build successfully https://griddataformats.readthedocs.io/en/latest/ ; if it continues to work on an open PR such as #170 then we can close this issue.