Issue #65: add generated API docs

[laurence-myers]

Generated API Docs are published to https://joiful-ts.github.io/joiful/

docs(readme): add links to API Docs (GitHub Pages)

feat(ci): add job to build and publish the generated API docs

• Use --frozen-lockfile when installing dependencies, in case the lockfile has not been updated.

I've set up CI as a separate workflow, but perhaps it should be a job in the existing workflow, so it we only publish docs when the normal tests etc have passed? (Since docs will only be published from master, I don't think it's a big issue, tests should have already been run on the pull request being merged in.)

feat(core): add typedoc, other supporting changes

• Ignore docs/ dir from Git
• Add NPM script typedoc
• Fix NPM script watch
• Export decorator interfaces in index.ts, so they appear in generated docs
• Add return types to all createXPropertyDecorator functions, using the decorator interfaces.
• Fix createBooleanPropertyDecorator using Boolean instead of boolean.
• Fix ObjectSchemaDecorator using boolean instead of object

TODO: update package-lock.json

Closes issue #65

[laurence-myers]

Looks like typedoc doesn't support NodeJS v10. Do we drop support for NodeJS v10, even though it only affects a dev dependency? Or, do we pass --ignore-engines to yarn install?

[laurence-myers]

Looks like typedoc doesn't support NodeJS v10. Do we drop support for NodeJS v10, even though it only affects a dev dependency? Or, do we pass --ignore-engines to yarn install?

I've resolved this by moving "typedoc" to its own sub-project/sub-directory. Now, docs/ contains a separate package.json, containing just typedoc. API docs will be generated into docs/dist/.

[laurence-myers]

Merged without review; better to have some imperfect API docs than none.