docs: improve alternative backend and client discoverability

[axelray-dev]

Fixes #484

What changed

Three minimal docs-only edits to improve search discoverability for alternative backends and alternative clients:

1. docs/how-to-guides/alternative-backends.md: Added a sentence after the intro paragraph mentioning searchable keywords ("self-hosted backend", "collector", "Jaeger", "Grafana Tempo", "OTLP-compatible server").

2. docs/how-to-guides/alternative-clients.md: Added language examples after "first-class supported languages": Go, Ruby, OpenTelemetry SDK. (Node.js and Rust removed from examples since they have first-class SDKs per docs/languages.md.)

3. docs/why.md: Replaced the vague "cross-language data integration and data export to any OpenTelemetry-compatible backend or proxy" with explicit links to the Alternative Clients and Alternative Backends pages, mentioning Node.js, Rust, Go, Jaeger, and Grafana Tempo.

Why this helps search/discoverability

The docs site uses Algolia search (alt-logfire-docs index). Users searching for "opentelemetry server", "self-hosted backend", "OTLP", "Jaeger", "collector", "Go", "Ruby", or "alternative clients" will now find these pages more easily because:

• The new keywords appear in the page body text (which Algolia indexes)
• The OpenTelemetry section in why.md now links directly to both pages, creating discoverable cross-references
• The language examples in alternative-clients.md match common search queries

Checks run

make lint          # passed, 243 files already formatted
make format        # 243 files left unchanged

Skipped checks

• No local MkDocs build (docs are rendered by the pydantic/unified-docs pipeline, not locally).
• No code changes. This is docs-only, so no tests were needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[cubic-dev-ai]

1 issue found across 3 files

Confidence score: 4/5

• This PR is likely safe to merge with minimal risk because the reported issue is documentation-only and does not indicate runtime or functional regression.
• The main concern is a user-facing docs inconsistency: docs/how-to-guides/alternative-clients.md says Node.js and Rust are outside first-class support, which conflicts with docs/languages.md and the page’s own SDK references.
• Given the moderate severity (5/10) but high confidence (9/10), this is worth correcting soon to avoid reader confusion about supported languages.
• Pay close attention to docs/how-to-guides/alternative-clients.md and docs/languages.md - ensure language support statements are consistent across docs.

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