docs: add API tokens and client keys + How to run the Unleash Proxy

[thomasheartman]

Also: do some minor cleanup in the proxy reference doc

This PR adds:

• A new "Unleash Proxy guides" section under how-to guides with a "how to run the unleash proxy" guide. More to come later.
• A new API tokens and client keys reference doc documenting the existing API tokens and client keys Unleash uses
• New capabilities to the docs' ApiRequest component (for use with the proxy now too 🆕)
• A little structuring and more information (all configuration options!) to the Proxy reference docs

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[thomasheartman]

How to run the Unleash Proxy

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟢	Statements	91.14%	5104/5600
🟡	Branches	79.68%	796/999
🟢	Functions	86.27%	1206/1398
🟢	Lines	91.07%	4988/5477

Test suite run success

845 tests passing in 121 suites.

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Name	Status	Preview	Updated
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[sighphyre]

Nice! Some small things I've commented on, feel free to ignore