Bump version to 0.3.0

Update site params to a nested map

config.toml: Update input_dir

templates/_default/index.gmi: Minor formatting nit

page: Ability to specify build tasks per page

Implements: https://todo.sr.ht/~adnano/kiln/15

templates/_default/atom.xml: Fix feed URL

Use io/fs

Use path/filepath instead of path

site: Expose root page to templates

site: Set URL for root directory

docs: Move functions to bottom

docs: Improve page documentation

page: Fallback to base template for page/index templates

Implements: https://todo.sr.ht/~adnano/kiln/22

Use github.com/google/shlex to parse commands

Implements: https://todo.sr.ht/~adnano/kiln/21

docs: Update frontmatter example

docs: Update example configuration

docs: Add note

docs: Update template variables

Preserve mode bits for static files

Fixes: https://todo.sr.ht/~adnano/kiln/25

docs: Tweak wording