reorganize documentation - add github pages

[erwindon]

Is your feature request related to a problem? Please describe.
Currently, all documentation is in one big README.md file.
This also mixes installation instructions, feature descriptions and release history.

Describe the solution you'd like
Continue on PR #455 to:

• separate the sections
• make the documentation available in github pages.
• provide the link to the documentation within SaltGUI

Describe alternatives you've considered

1. separate sections and generate pages locally
   this complicates the release procedure and does not make the documentation globally accessible. therefore this alternative is rejected.

Additional context

• thanks to @bsikar for suggesting this
• duplicate documentation must be prevented, do not just copy and reformat the documentation.
• might use docsifyjs
• see also https://erwindon.github.io/SaltGUI/ for first results

[erwindon]

this PR will be limited to making the README.md file visible as a github-page.

• move all documentation to directory docs
• setup GitHub to use directory docs to generate the github-page (first on this branch, later on master, see https://github.com/erwindon/SaltGUI/settings/pages)
• add icon to SaltGUI page header, which links to the github-page

to do in a new PR:

• create separate section for (now reduced) README.md file, the user manual section and the release history section. include an index; and
• add add other files (CONTRIBUTING.md, LICENSE.md and PERMISSIONS.md), update index to match; and
• separate the use manual section into chapters for each page. update the index to match; and
• update the documentation icon on each page to use a relevant deep-link into the documentation.
  see also reorganize documentation - additional improvements #469

[sonarqubecloud]

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