fix: fall back to debug.ReadBuildInfo for version when ldflags not set

[znd4]

Problem

When helm-docs is built via go install (e.g., by pre-commit's language: golang hook type for the helm-docs-built hook), no -ldflags are passed, so the version variable remains an empty string. This causes:

1. helm-docs --version prints helm-docs version with no version
2. The README footer (Autogenerated from chart metadata using [helm-docs vX.Y.Z]) is not rendered, because the default template checks {{ if .HelmDocsVersion }}

This means the helm-docs-built pre-commit hook produces different output than the release binary, causing CI failures when the committed README was generated by a release binary (or vice versa).

Fix

Add a fallback using runtime/debug.ReadBuildInfo() in an init() function. When Go builds a binary via go install module@version, it embeds the module version in the binary metadata. This is a stdlib-only change with no new dependencies.

• The ldflags-injected version always takes priority (guard clause returns early)
• The v prefix from Go module versions is stripped to match the existing convention (the footer template already prepends v)

Reproduction & validation

A test workflow on the fork validates both paths:

• go install ./cmd/helm-docs (no ldflags): version is set, footer renders ✅
• pre-commit helm-docs-built hook: footer renders, output is idempotent ✅