website: add example curl command to test Beacon API endpoint in blsync.md

[shazam8253]

This adds a short example section showing how to use curl to verify that a Beacon API endpoint is reachable and returns valid light client headers. Helps developers quickly test endpoint connectivity before integration.

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[tr1sm0s1n]

I see this endpoint belongs to the nimbus client. The docs are solely for geth related utilities.

[lightclient]

Thanks Shanti! What I'm hoping for here in the example is more of a one line command that gets the checkpoint and starts geth using blsync with that root.

It should look something like this

export BEACON=<server URL>
geth --beacon.api=$BEACON --beacon.checkpoint=$(curl <rest of command to get the check point root>)

You'll want to use jq to access the exact values from the JSON response from the beacon api.

[lightclient]

Started leaving a few comments, but decided to push a few commits instead. Mostly I think the checkpoint URL isn't needed here since the finalized block can be determined from the light sync endpoint. I also reordered things a bit!

[lightclient]

Suggested change

	This will automatically fetch the latest finalized checkpoint and launch Geth in snap sync mode with light client support (blsync).
	This will automatically fetch the latest finalized checkpoint and launch Geth with blsync enabled.

Since snap sync has long been default, I don't think we need to mention explicitly. Also updating to specify blsync explicitly since it is already defined as a light client mode.

[lightclient]

I don't think you need to separate these two servers. The light sync server will also be providing the standard beacon API so you can query eth/v1/beacon/headers/finalized