docs: Update receive.md for FCM doc revamp

[jadenlin-g]

Description

Updating this page as part of the FCM doc revamp project. This includes some voice edits and adding new get started content for users.

Related Issues

N/A

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

[cynthiajoan]

nit: the first few lines

Project: /docs/cloud-messaging/_project.yaml
Book: /docs/_book.yaml
page_type: guide
{% include "_shared/apis/console/_local_variables.html" %}
{% include "_local_variables.html" %}
{% include "docs/cloud-messaging/_local_variables.html" %}
{% include "docs/android/_local_variables.html" %}

looks wired in MD view also not related to the docs here. Probably should remove.