Resolves https://github.com/webrecorder/browsertrix/issues/2073
### Changes
- Removes "URL List" and "Seeded Crawl" job type distinction and adds as
additional crawl scope types instead.
- 'New Workflow' button defaults to Single Page
- 'New Workflow' dropdown includes Page Crawl (Single Page, Page List, In-Page Links) and Site Crawl (Page in Same Directory, Page on Same Domain, + Subdomains and Custom Page Prefix)
- Enables specifying `DOCS_URL` in `.env`
- Additional follow-ups in #2090, #2091
