docs: reconstruct examples for full godoc coverage; fix stale package doc

[mpyw]

Reconstructs the godoc examples and fixes an outdated package comment.

• doc.go: the package comment still described the pre-explicit-model design (a /*> */ partial include, "drops clauses that become empty and AND/OR left dangling"). Rewritten to describe the current explicit model and @include.
• example_test.go: rebuilt as 26 testable examples. Package-level examples (Example, Example_conditional, Example_iteration, Example_literal, Example_inListExpansion, Example_arrayBind, Example_include, Example_dialects) cover the README concepts and render in the package's top Examples section; the rest attach to their public symbol so they render under it (ExampleParse, ExampleParseFile, ExampleExpand, ExampleExpandFile, ExampleNewParser, ExampleWithDialect, ExampleWithLoader, ExampleWithStackedLoader, ExampleWithEvaluator, the loader constructors, ExampleParser_Parse/_ParseFile, ExampleTemplate_Build, ExampleRegistryLoader_Register, ExampleStatement_SQLWithArgs). Each statement-building example prints SQL, Args, and the values-embedded SQLWithArgs form. Fixes the old lowercase ExampleStatement_sqlWithArgs (now attaches to the method page).
• bench_test.go: remove the micro-benchmark (not run in CI, stale employees schema); keep its concurrency/immutability check, moved to bisql_test.go as TestTemplateConcurrentBuild (run under -race).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.52%. Comparing base (3cc7fa8) to head (5c4ba10).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #21   +/-   ##
=======================================
  Coverage   96.52%   96.52%           
=======================================
  Files          11       11           
  Lines         951      951           
=======================================
  Hits          918      918           
  Misses         21       21           
  Partials       12       12           

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