[docs](trx-rs): generate the example config and correct the manual

trx-rs.toml.example was maintained by hand and had fallen well behind: no
[[rigs]], no [[remotes]], no [timeouts], no bandplan or decode-history
settings, and a [frontends.http].default_rig_id that had been renamed.

Generate it from the config structs instead, so a new field shows up the moment
it exists, and add a test that fails when the checked-in copy drifts:

    cargo run -p trx-config --example generate_example

Section comments come from a small table; a section without an entry is still
emitted, so forgetting a comment can never drop a setting from the example.

The manual was wrong about the basics.  It listed five config search paths, none
of which the loader has ever looked at (the real order is ./trx-rs.toml → XDG →
/etc), called --print-config output "fully commented" when it carries no
comments at all, and documented a TRX_PLUGIN_DIRS variable no code reads.  It
also still described [frontends.rigctl].port as the bind port years after
rig_ports replaced it.  Fixed, and the new configuration features are written
up alongside.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SyX26FCpMQxiBoC7r5K1A7
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-config/Cargo.toml

@@ -18,6 +18,7 @@ trx-core = { path = "../trx-core" }
 trx-decode-log = { path = "../decoders/trx-decode-log" }
 trx-reporting = { path = "../trx-reporting" }
 serde_ignored = "0.1"
+toml_edit = "0.22"
 
 [dev-dependencies]
 tempfile = "3"