Add Documentation about Adminer, and deprecate pgAdmin

[alexnuttinck]

Is your feature request related to a problem? Please describe.

Adminer has been added to the FADI stack (See Issue #89). (https://www.adminer.org/ supports MySQL, MariaDB, PostgreSQL, SQLite, MS SQL, Oracle, SimpleDB, Elasticsearch, MongoDB)
Now we need to update the doc (install, userguide, slides) to deprecate pgadmin.

Checklist

• Update the install script (update helm-fadi), and the documentation related to.
• Update the userguide.
• Update the different slides and presentations. (https://fadi.presentations.cetic.be , ...).
• Update the install script, and the documentation related to.

[alexnuttinck]

pgadmin is now removed from helm-fadi (chart 0.1.19).

[alexnuttinck]

pgadmin repo is now archived.

[alexnuttinck]

I made a PR #97 where I updated the userguide.

[banzo]

@fzalila do the test/use cases need to be adapted accordingly?

[fzalila]

@banzo
For test cases here, some test cases should be updated (TC-5, TC-6, TC-7 and TC-8).
For implemented tests, the pgadmin test is not so advanced. So removing it and replacing it with Adminer tests should be easy to do.

[alexnuttinck]

Ok. I think we can close this issue now.