move explanation from frontend user's guide

[keithmanville]

To improve the structure of the documentation, move the descriptive information from the existing frontend user's guide to a new standalone document. This will make the information more readily available. The two documents should refer to each other.

Definition of Done

• A new document is created explaining the different Dioptra resources using the explanations available in the frontend user's guide.
• Once the new document is created, remove the information from the frontend user's guide to simplify it.
• Both documents refer/link to each other.