docs: add info for webhook signature validation

[brad-dow]

Adding content for optional but recommended webhook signature validation

Fixes #3022

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[mkurapov]

Suggested change

	Finally, compare the signature in the header to the expected signature you generated. For security, use a constant-time comparison function to prevent timing attacks. Also, check the timestamp to ensure that it is within the allowed TTL (configured in the `ADMIN_API_SIGNATURE_TTL_SECONDS` environment variable) to ensure freshness.
	Finally, compare the signature in the header to the expected signature you generated. For security, use a constant-time comparison function to prevent timing attacks.

Does not apply to webhooks for now

[mkurapov]

Version is set using SIGNATURE_VERSION

[mkurapov]

Suggested change

	```
	// Really cool code example in some commonly used language (JavaScript, Python?)
	// Extract timestamp and signatures from header
	// Prepare the signed payload string
	// Generate the expected signature
	// Compare the signatures and check the timestamp
	```
	```js
	function verifyWebhookSignature(request: Request): boolean {
	const signatureParts = request.headers['Rafiki-Signature'].split(', ')
	const timestamp = signatureParts[0].split('=')[1]
	const signatureVersionAndDigest = signatureParts[1].split('=')
	const signatureVersion = signatureVersionAndDigest[0].replace('v', '')
	const signatureDigest = signatureVersionAndDigest[1]
	if (signatureVersion !== config['SIGNATURE_VERSION']) {
	return false
	}
	const payload = `${timestamp}.${canonicalize(request.body)}`
	const hmac = createHmac('sha256', config['SIGNATURE_SECRET'])
	hmac.update(payload)
	const digest = hmac.digest('hex')
	return digest === signatureDigest
	}