feat(changelog): move announcements page data into markdown from JSON

[contolini]

Makes our announcements page more maintainable by storing its content in markdown instead of JSON.

Changes

• Replaces our JSON announcements file with a markdown version.

Testing

1. Pull down this branch.
2. yarn && yarn start
3. Visit http://localhost:3000/updates-notes and everything should look like https://ffiec.cfpb.gov/updates-notes

Notes

• The markdown changelog was added in a previous PR and already exists in master. We'll delete the JSON version in a future PR once we're sure everything works as expected.
• useRemoteMarkdown.jsx is a copy of useRemoteJSON.jsx with a few minor changes. Another option would have been to rename the file to something generic like useRemoteFile.jsx and do filetype checking in the hook to return either JSON or markdown. Lmk if you'd prefer that method.