Provide quick introduction at start screen

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The start screen should contain a quick introduction in one page. Requires #371. Possible content:

• Welcome
• Link to manual
• Select and browse source- and target-vocabularies left and right
• Select a concept to get mappings and mapping-recommendations from this concept in the Mapping-Navigator
• Mappings are stored in X different databases. Please login for write-access!
• Stored mappings can be queried via the Search
• blala about concordances
• Link to Feedback/project page

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The "User Interface" tab of settings could also contain more explanation text, e.g. a list of common icons and their meanings (settings, data source, filter, favourite...).