♻️ Change NeedsBuilder format to needs

[chrisjsewell]

Changing the format from json to needs will make it more specific to target the NeedsBuilder, and any derivatives such as in #960, #961, #962

The format attribute of the sphinx builder is perhaps a bit misleading, since it is mainly used to target a "class" of builders, rather than a specific output format,
see: https://www.sphinx-doc.org/en/master/usage/builders/index.html

This will allow for logic in other parts of the sphinx-needs pipeline, e.g. to skip certain build steps not required by the needs builders.

[danwos]

Thanks for the PR 👍 Looks good.