Suggestions for initial documentation

[davidfromdk]

This PR is branched out from #2 and contains the following suggestions:

• Correction of spelling errors and a broken link.
• Suggestions for rephrasing of sentences and restructuring of content.
• Replacement of "3dsecure.io" with "3-D Secure Server", since 3dsecure.io also includes an MPI product. Furthermore, it makes it easier to integrate the MPI docs into this documentation when time allows it.

Some suggestions not included in this PR.

• Add API endpoints to the authentication flow figure mentioned in the text below the figure.
• Open external links in a new tab/window.
• The Getting Started could benefit from explanations of the different parties involved, e.g. Issuer (ACS), Requestor, Cardholder.
• Improve styling. Styling is a bit off various places e.g. links are not highlighted with a color, spacing in lists are off.
• The sidebar menu is a bit confusing due to the items being highlighted and indented, e.g. if clicking on: https://clearhaus.github.io/3DSv2-api-documentation/preauth.html the following is also highlighted: 3.2. /auth endpoint and 3.3. /postauth endpoint.

Feel free to use what makes sense :-)