Improve instruction texts

[lukpueh]

Below some general notes about instructive texts in the tool that were gathered from user tests:

• Less text in general
• More (concise) text about why someone would want to use in-toto
• More contextual help (maybe even about supply chain tools, e.g. how to find git's remote URL)
• Fix grammatical/syntactical issues
• Revise punctuation
• Emphasize jargon terms in markup (e.g. in-toto, layout, functionaries)
• Some rephrasing suggestions (on start page)
  "Create in-toto layout" --> "Create an in-toto layout"
  "your clients can be sure that"--> “your clients can check that"
  "An in-toto layout defines" --> "An in-toto layout models/describes,..."

[lukpueh]

• More rephrasing suggestions
  Make labels (above input fields) imperative Initial PR (complete layout tool) #14 (comment)

[lukpueh]

@ultrasaurus pointed out that that "It would be nice to see one path laid out as a guide or tutorial".

See also a response in in-toto/specification#15 (comment).