Cleanup API spec, documentation and adjust naming of things

docs/changelog.md

docs/details/authentication/sender.md

docs/getting-started/sending/metadata.mdx

docs/getting-started/sending/overview.mdx

docs/more/glossary.mdx

docs/status-and-error-codes.md

spec/endpoints/attachments/uuid.yml

spec/endpoints/destinations/index.yml

spec/endpoints/destinations/uuid.yml

spec/endpoints/info/get-info.yml

spec/endpoints/submissions/index.yml

spec/endpoints/submissions/uuid.yml

spec/examples/create-submission.yml

spec/examples/submission-created.yml

spec/examples/submission-state.yml

spec/examples/submission.yml

spec/parameters/attachmentId.yml

spec/parameters/submissionId.yml

spec/schemas/announced-content-structure.yml

spec/schemas/application-state.yml