Improved readme and docs

[etoledom]

Closes GRA-151

Description

Improving README.md, articles, and inline docs.

Testing Steps

• Read the new README.md and check that things are well written.
• Build the swift-docc bundle (Product -> Build Documentation)
  • Check that all links work.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Gravatar Prototype Build by scanning the QR code below to install the corresponding build.

	App Name	Gravatar Prototype Build
	Build Number	2418
	Version	PR #778
	Bundle ID	com.automattic.gravatar-sdk-demo-uikit.prototype-build
	Commit	4cdba05
	Installation URL	2tv1skef6ev90

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[pinarol]

Looks great 🎉