[PR] docs: add `declarations` page to config
[PR] docs: add `declarations` page to config

Summary:
Hi,

`[declarations]` is missing in the page config. Hence no TOC, no title.
https://flow.org/en/docs/config/declarations
Pull Request resolved: https://github.com/facebook/flow/pull/7403

Differential Revision: D13858450

Pulled By: mroch

fbshipit-source-id: ef202b5449be9d257bc281af2f4666bd70a76e84

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow