[DOC] Initial Documentation V3

## Topics

- [x] API security
- [x] Persistence
- [x] Invoking workflows
- [x] What is new in Elsa 3
- [x] Feature overview
- [ ] Activity behaviors
- [x] Workflow variables
- [x] Expressions
- [ ] Workflow runtimes
- [x] API endpoints
- [ ] Composite Activities
- [ ] Tasks & Jobs
- [x] Triggers
- [x] Bookmarks
- [x] Activities
- [x] Workflows
- [x] Input and output
- [x] Programmatic workflows vs designed workflows

## Quickstarts

- [x] ASP.NET application - workflow server
- [x] ASP.NET application - dashboard
- [x] ASP.NET application - workflow server + dashboard (aka "monolith")
- [x] Console app

## FAQ
- [ ] Will a blocking activity block the entire workflow?
- [ ] How does Elsa compare to other .NET based workflow solutions?

## Guides

- [ ] Timer-triggered workflows
- [x] Writing custom activities
- [ ] Activity type providers
- [x] Integrating with external applications
- [ ] Hosting in a cluster
- [x] HTTP triggered workflows