docs: clarify partial local server configs

[sdairs]

Stacked on #756 (docs/755-readme-products).

local server start --config was described as a custom config file at the point of use, making it easy to assume a complete default config was required. The flag and empty config-list guidance now explain that files contain only changed settings and inherit the remaining defaults.

The README demonstrates server and profile settings in one overlay, explains how to use an existing config.d fragment, and documents source edits, restart selection, generated output, and file lookup. Help keeps the short operational guidance; config loading and JSON output are unchanged.

Closes #764.

Validation (run before restacking; patch unchanged):

• cargo fmt --all and git diff --check
• cargo clippy -p clickhousectl -- -D warnings
• cargo test -p clickhousectl: one existing port-dependent stderr assertion fails (details below); all other executed tests pass, with one ignored test.
• cargo test -p clickhousectl --test local_version_error_test --test telemetry_test covers the binaries the full run stopped before reaching.
• cargo check -p clickhousectl --no-default-features
• cargo clippy -p clickhousectl --all-targets --no-default-features -- -D warnings
• Manually reviewed rendered server/start/configs help against the repository help standard.

Local test limitation: config_and_argument_failures_carry_their_full_human_detail_in_json fails because OrbStack already listens on 8123 and 9000. The CLI emits Note: default ports in use, auto-assigned HTTP:8124 TCP:9001 before the missing-config error, while the unchanged test expects only the error. A focused rerun reproduces it. The change does not touch port selection or error rendering.

Restacking validation: git range-diff confirms the same single-commit patch; cargo fmt --all and git diff --check pass on the new base.

[iskakaushik]

LGTM