improve `addArnGenerator()` templating in APM Lambda docs to cope with failed api.github.com request

[trentm]

From https://github.com/elastic/apm-agent-nodejs/pull/2630/files#r840031451 discussion: The lambda-selector in docs/lambda-selector/lambda-attributes-selector.asciidoc and its templated usage in the Java, Python, and Node.js docs will result in slightly confusing getting started docs if the api.github.com request fails. We could probably template this better so that the content is more helpful (clearer unreplaced template vars, links to the 'releases' tables) when it isn't replaced.

Also I don't know if the same applies when someone is browsing with JS disabled. Or for a11y.