Add command to launch ota from declarations folder

[martinratinaud]

Up until now, if the user wants to generate the snapshots and versions, he is obliged to launch npm start Service from the OpenTermsArchive project and setup a config/development.json file pointing to the declarations folder he wants to use. Something like:

{
  "services": {
    "declarationsPath": "../sandbox-declarations/declarations"
  },
}

This would result in files created in OpenTermsArchive/data folder.

While this works well, it obliges user to clone both OpenTermsArchive and declaration repo.

In most of use cases, a user will work on one instance, checkout the corresponding declarations folder and work from there.

This PR aims at providing a new command ota that will be exposed as a bin command and that will generate the snapshots and versions directly in the folder it is called from.

[martinratinaud]

@Ndpnt can you please review before I make changes so that I come back to this only once. Thanks

[Ndpnt]

Avoid commit message mentioning contributor's review message and explain the intention instead: Add Nico's suggestion on doc -> Improve documentation

[martinratinaud]

I plan to merge this tomorrow.