docs: fix upstream target syntax in examples

[raffaelschneider]

Summary

Fixes a long-standing documentation bug surfaced in zentinelproxy/zentinel#254: the quickstart and most other config examples used targets { target { address "X" } }, which neither the single-file nor the multi-file KDL parser actually accepts. Users following the quickstart hit Upstream '...' requires at least one target and had to reverse-engineer the working form.

This PR switches all examples to the form the single-file parser at crates/config/src/kdl/upstreams.rs reads:

upstream "backend" {
    target "host:port"
    target "host:port" weight=2
}

— which is also what many examples in the same docs were already using, so the docs were internally inconsistent (e.g. quick-start.md had both forms on the same page).

Scope

• 36 files touched across getting-started, configuration, concepts, operations, deployment, development, examples, reference, and appendix.
• Versioned archives under content/v/25.12, 26.01, 26.02, 26.04, 26.05 are intentionally not changed — those releases actually shipped with the broken syntax in their docs, and rewriting them would falsify the historical record.

Transformations

Before	After
target { address "X" }	target "X"
target { address "X" weight=N }	target "X" weight=N
Multi-line target { address "X"; weight N; max-requests N }	target "X" weight=N max-requests=N
targets { ... } wrapper	unwrapped (single-file parser ignores it)
targets { } example in directives.md	replaced with a meaningful target "127.0.0.1:8080"
target { weight=N } row in HAProxy comparison table	target "host:port" weight=N

Helm/Jinja2/envsubst templates in config-management.md and Rust test format-strings in integration-tests.md were also corrected so the templates produce valid KDL.

Not in this PR

Issue #254 has two more reports that are not documentation bugs and need separate fixes in zentinelproxy/zentinel:

1. /var/log/zentinel permission failure when the official Docker image runs as non-root — needs a container/process fix.
2. Metrics listener at metrics.address "0.0.0.0:9090" never binds — needs investigation in the proxy.
3. The user's open question about running two listeners with separate route sets is also out of scope here.

There is also a deeper underlying issue: the single-file parser (crates/config/src/kdl/upstreams.rs) and the multi-file parser (crates/config/src/multi_file/parsers.rs) accept different target syntaxes. That divergence is what made it so easy for docs to drift from reality. Worth a follow-up tracking issue in the proxy repo.

Test plan

• Render the docs site locally and spot-check getting-started/quick-start.md, configuration/upstreams.md, configuration/namespaces.md, getting-started/installation.md, deployment/docker.md.
• Copy the quickstart KDL into a zentinel.kdl and confirm zentinel --config zentinel.kdl --check (or --dry-run) succeeds against a current build.
• Grep content/ (excluding content/v/) for any remaining target { or targets { — should be zero.

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