Docs: External Connection via WordPress.com Application

[jeffpaul]

Description of the Change

This PR folds the standalone (and often not seen/read) wpdotcom-authorization.md file's contents into README.md and attempts to update the docs to current integration steps. This PR is currently a DRAFT as I'm unable to successfully create an External Connection via the WordPress.com Application method, though this may be due to testing this with a local site (I'll next try with a "live" site).

Alternate Designs

Keep as-is, though having these sorts of instructions outside the README.md seems suboptimal (at least in the case where there's no dedicated docs site).

Benefits

Ensures that folks going through the External Connections setup steps in README.md also have details on doing so via the WordPress.com Application method.

Possible Drawbacks

None identified.

Verification Process

Working on verifying myself, will update here once that's successful.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

n/a

Changelog Entry

### Updated
- Docs on Creating External Connections via WordPress.com Application (props @jeffpaul)

[jeffpaul]

Noting that currently trying to connect a local single WordPress instance running WP 5.7.2 and Distributor 1.6.4 to a WordPress.com Business Plan site gives the following message:

Limited connection established.

• Authentication failed due to invalid credentials.
• Push distribution unavailable.
• Pull distribution limited to basic content, i.e. title and content body.

Attempting to connect a public multisite instance running WP 5.7.2 and Distributor 1.6.4 gives the following message in the OAuth2 authorize flow:

Invalid request, please go back and try again.
Error Code: invalid_request
Error Message: Mismatch in redirect_uri.

[jeffpaul]

Ah right, forgot to update the WPCOM app to my "live" test site, after doing so I get the same response as testing locally:

Limited connection established.

• Authentication failed due to invalid credentials.
• Push distribution unavailable.
• Pull distribution limited to basic content, i.e. title and content body.

[jeffpaul]

Potentially related/useful: https://github.com/Automattic/client-example

[github-actions]

@jeffpaul thanks for the PR! Could you please rebase your PR on top of the latest changes in the base branch?