Parameterise Usage Help of CliApplication

## Problem
Currently the following places are hard coded:
```
val BaseDocUrl = "https://ergoplatform.github.io/ergo-tool/api"
```
```
Usage Syntax:\tergo-tool $name ${cmdParamSyntax}
```

Thus:
- api url shown by HelpCmd command is incorrect for dex commands
- usage syntax have incorrect name of executable

## Solution
- [ ] transform BaseDocUrl into abstract method `def baseDocUrl: String` in CmdDescriptor
- [ ] https://github.com/ergoplatform/ergo-dex/issues/7
- [ ] introduce `def launcher: String` method to use in Usage Syntax above.