DOCSENG-135 - reduce large api files

[davidejones]

What does this PR do? What is the motivation?

• Use a new partial called api/load-specs.html which loads the spec or retrieves already loaded spec from hugo store
• When reading specs in hugo try to use the yaml file over the deref
• Delete full_spec_deref.json files
• Only generate full_spec_deref.json files during the build not committed
• move spec yaml files to assets dir for possibility of using openapi parsing

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

https://docs-staging.datadoghq.com/david.jones/docseng-135/api/latest/