docs: add list of public endpoints to getting started page

[wakamex]

Motivation

I found it frustrating not being able to start up a Lodestar node without having to google for public endpoints, which actually sent me to the docs of another CL client. Having to do that is much more error-prone than following a link from your docs.

Description

I added a link to the same public endpoints page linked from the docs of another major CL client.
I don't think an additional at your own risk is necessary here, since this link is already in the warning block that starts off with Please use this option very carefully (and at your own risk).

[CLAassistant]

All committers have signed the CLA.

[nflaig]

Makes sense to add this link, much better than having people use google to find it.

Thanks @wakamex

[codecov]

Codecov Report

Merging #6644 (c9ca2a0) into unstable (f2ec0d4) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff            @@
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  Coverage     61.69%   61.69%           
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  Files           556      556           
  Lines         58820    58820           
  Branches       1887     1887           
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  Hits          36287    36287           
  Misses        22492    22492           
  Partials         41       41           

[wemeetagain]

🎉 This PR is included in v1.18.0 🎉