Add project_id to the Note API response

What does this MR do and why?

Adds project_id to API::Entities::Note so that the note's project is known in the API response.

This will be the response for the following APIs: search, notes, discussions all of which can benefit from having project_id included. Epic Notes don't have a project so project_id will be null.

Also updates the documentation.

Screenshots or screen recordings

Example response

{
  "id": 48122,
  "type": null,
  "body": "...",
  "attachment": null,
  "author": {
      "id": 1,
      "username": "root",
      "name": "admin",
      "state": "active",
      "avatar_url": "https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80&d=identicon",
      "web_url": "http://127.0.0.1:3000/root"
  },
  "created_at": "2011-12-22T23:09:14.000Z",
  "updated_at": "2011-12-22T23:09:14.000Z",
  "system": false,
  "noteable_id": 7359,
  "noteable_type": "Issue",
  "project_id": 22,
  "resolvable": false,
  "confidential": false,
  "internal": false,
  "noteable_iid": 227,
  "commands_changes": {}
}

How to set up and validate locally

1. Do an API search for notes. E.g. /api/v4/projects/22/search?scope=notes&search=something.
2. Note that the project_id is in the response.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #382183 (closed)