chore: add usage docs for seo

[ayushnirwal]

What

Adds documentation related to Seo systems.

Related Issue(s):

https://github.com/rtCamp/headless/issues/431

How

• Adds example showing how to register a plugin.
• Adds a section explaining requirements for a plugin to be registered.

Checklist

• I have read the Contribution Guidelines.
• My code is tested to the best of my abilities.
• My code passes all lints (ESLint, tsc, prettier etc.).
• My code has detailed inline documentation.
• I have added unit tests to verify the code works as intended.
• I have updated the project documentation as needed.
• I have added a changeset for this PR using npm run changeset.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 8167546

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[coveralls]

Pull Request Test Coverage Report for Build 15050720227

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 60.354%

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Totals
Change from base Build 15050382275:	0.0%
Covered Lines:	507
Relevant Lines:	743

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💛 - Coveralls

[justlevine]

I think we should provide the actual shape, if not an example of these. See the level of fidelity in query-engine.md

[justlevine]

A bit more on what needs to be provided to registerPlugin, and we should be good to merge.