document 0.3 changes and finalize release

[pietroppeter]

edit: consolidated a few comments with todo edit 2: changed my mind again, release of 0.3 will require only to document past changes in order to be completed

having completed the breaking change:

• [x] make codeAsInSource the default and codeFromAst an option, see #94

the only really required thing in order to release 0.3 is:

• [ ] document new stuff (and also stuff from 0.2 minor versions), see #81 and recent PRs
  • [ ] added a new newNbSlimBlock and newNbCodeBlock
  • [ ] nbText now does not read code
  • [ ] new nbTextWithCode that does read code
  • [ ] config utils for a theme such as nimibook
  • [ ] trim readme and it will NOT to depend on index.nim
  • [ ] add to readme a deploy section which documents how to deploy with GitHub pages
  • [ ] make links in docs relative so that they work in deploy preview (already done in js/karax pr, see)
  • [ ] nbRawOutput

all this other stuff can be safely added in 0.3.x updates, I will create issues for each one of them and grouped them in a project/milestone.

• [ ] put back highlighting js #68
• [ ] put back untyped nbFile (see discussion in #34)
• [ ] add test for nbFile
• [ ] add test for nbImage
• [ ] add also nbAudio and nbVideo (with tests) and a media example
• [ ] add nbShell (wraps osproc.execCmdEx) (completes #34)
• [ ] add a document that shows how to use nbPython and maybe offers nbPythonCode implementation (see this commit) as a novelty (and also as a way to show how to do a custom code block)
• [ ] rethink/review markdown related stuff (both for markdown backend and for converting to markdown) and in general change the documentation:
  • [ ] readme.md will not be generated anymore from nim (too clumsy to fix a small typo in readme) - instead index.nim might depend on readme (partial inclusion).
  • [ ] add a md2html.nim file that converts a given md file to html using default theme (as it happens in nimibook)
  • [ ] this same md2html file will also be saved with markdown backend as md2html.md
  • [ ] and md2html.md will be converted itself as a md2html2md2html.html (which might be slightly different from md2html)
• [ ] config file found should report path relative to initDir
• [ ] NbBlock as a container: I think this can be done without breaking changes and it opens up many possibilities of use cases.

[pietroppeter]

updates on stuff to do:

• [ ] put back untyped nbFile (see discussion in #34)
• [ ] trim readme not to depend on index.nim
• [ ] add to readme a deploy section which documents how to deploy with GitHub pages
• [ ] make links in docs relative so that they work in deploy preview

[pietroppeter]

• [ ] make codeAsInSource the default and codeFromAst an option, see #94
• [ ] add a document that shows how to use nbPython and maybe offers nbPythonCode implementation (see this commit) as a novelty (and also as a way to show how to do a custom code block)