docs: document the OpenTelemetry export and add a New Relic guide (#14557)

Langflow has exported OTLP traces, metrics and logs for a while and none of it
was documented. Nothing in docs/ mentioned OTEL_EXPORTER_OTLP_ENDPOINT, so the
feature was discoverable only by reading the source. The existing Instana page
covers the Traceloop LLM-tracing route, which is a different thing: that one
carries prompts and completions on purpose, this one deliberately does not.

Two pages, because the vendor guides all need the same base and Instana is next.

The OpenTelemetry page covers what actually gets exported, with the attribute
set and the real protocol values read off the code rather than the plan, and
what does not: prompts, completions, exception messages, database bound
parameters. It names the two settings that widen that boundary, since an
operator should not discover LANGFLOW_OTEL_LOG_BODIES from a support ticket.
It also documents span volume and the LANGFLOW_OTEL_DB_SPANS trade, with the
measured numbers, so nobody meets that on an invoice.

The New Relic page carries the vendor specifics: the ingest license key (and
why a user API key is the wrong one), gzip for the 1 MB cap, and delta
temporality, verified by checking the exporter's preferred temporality with and
without the variable rather than assuming the SDK honours it.

Its verification section is the part worth keeping. A 2xx from New Relic is not
evidence, because it acks and then discards invalid data during async
validation, so the page gives the NRQL to confirm the data is really there,
including the NrIntegrationError check. Both traps we actually hit are written
down: the "required metrics are missing" banner is the curated view matching
semantic conventions rather than an ingest failure, and an open-ended time
window measures ingest lag and reads as 30% data loss.

Cross-links between the two new pages are relative rather than absolute, because
absolute slugs resolve against the released docs version where these pages do
not exist yet, and the build rejects them.