Add the initial website files.

Jekyll is used to generate the website from the documentation files.
When ready, the generated output will be pushed to GitHub Pages.

Progress on #47.
Merge pull request #67 from nicksay/web

Add the initial website files.
Consolidate all state keys into a single enum.
Merge pull request #68 from nicksay/state-keys

Consolidate all state keys into a single enum.
Add experimental "unified execution" for scripts and styles.

Split inline script and style evaluation into two functions:
- `eval`: conditionally evaluate the text only if changed
- `exec`: unconditionally execute the text

This matches the behavior of external script and style installation:
- `load`: conditionally install the resource only if changed
- `get`: unconditionally install the resource.

For now, guard the new logic behind a `experimental-execute-unified` config.

Progress on #25.