feat: CLI flags to override docs_dir and output_dir

[engineervix]

Problem

docs_dir and output_dir are currently only configurable via kwelea.toml. This means:

• CI/CD pipelines that want to output to a platform-specific directory (public/, dist/, _site/) must either edit the config file or maintain multiple config files.
• Monorepo setups that keep docs in a non-default location need a custom config committed for every variant.

Proposed Solution

Add CLI flags to kwelea build and kwelea serve that override the config values at runtime:

kwelea build --output public
kwelea build --docs content/docs --output dist
kwelea serve --docs content/docs

Flag	Overrides	Commands
--output / -o	build.output_dir	build
--docs	build.docs_dir	build, serve

Flags are purely additive — they override the config value when set but do not require the key to be absent from kwelea.toml. An unset flag leaves the config value unchanged.

Implementation Notes

The change is contained to the command layer:

1. Register flags in cmd/build.go and cmd/serve.go via cmd.Flags().StringP(...)
2. After config.Load(cfgFile), check if each flag was explicitly set (cmd.Flags().Changed("output")) and override cfg.Build.OutputDir / cfg.Build.DocsDir accordingly
3. The rest of the pipeline (nav.NewSite, builder.Build, server.Start) already threads through the config struct — no deeper changes needed

Acceptance Criteria

• kwelea build --output <dir> uses <dir> as output, ignoring kwelea.toml's output_dir
• kwelea build --docs <dir> uses <dir> as docs source, ignoring kwelea.toml's docs_dir
• kwelea serve --docs <dir> uses <dir> as docs source for the dev server
• Flags are documented in --help output with clear descriptions
• Config file value is used unchanged when the flag is not passed