ci: auto-deploy sandbox to github pages

feat: add wasm-based djoc sandbox

feat: support `add_title` on html output

feat: add standalone toggle to latex rendering

feat: add `add_title` setter

feat: add standalone option to html builder

build: add feature toggles

fix: link definitions

feat: add subcaption as default package

fix: en dash and em dash

chore: bump to jotdown 0.3.0

docs: update README

fix: skip non-parsable manifests

refactor: default build_dir value is None

feat: add `add-title` field to builder manifest

refactor!: move locale to builder config

fix: improve compile command

test: document removal of doctest for write_pdf

fix: tests for use in Nix shell

fix: properly filter source for Nix