fix(docs): don't merge payloads for PUT requests

[kanadgupta]

🚥 Fixes #738

🧰 Changes

For some background, here's the general workflow for when syncing a Markdown file to ReadMe via the rdme docs (or changelogs/custompages) command:

1. Grab the slug from the page
2. Make a GET request to the ReadMe API to confirm that the page exists
3. If the page exists, update the page via a PUT request
4. If the page does not exist, create the page via a POST request

The problem in step 3 is that we take the response body from the GET request (called existingDoc in the code below) and merge that with the Markdown file's data when making the PUT request:

rdme/src/lib/syncDocsPath.ts

Lines 107 to 111 in d311af9

	body: JSON.stringify(
	Object.assign(existingDoc, {
	...payload,
	})
	),

This isn't really necessary since the API already merges the PUT request body with the existing doc, and it's creating adverse side effects when our API expects one field and not another (see #738 for an example).

🧬 QA & Testing

Do tests pass?

[garrett-wade]

Looks good, had the exact same code in a branch ready to go :)