Add a bunch of articles in markdown format

This commit contains a "raw" markdown version of each original HTML article. The main issues with the md files are:

1. The md formatting is the default one generated by pandoc... I don't know if a different md flavour should be used for the project (?)
2. The code blocks are left-aligned, so they look ugly in the md source. This is a side effect of the HTML preprocessing step. I don't know if this could be a major problem :(
3. External link and image URLs are not processed, so they will point to wrong locations.

Regarding 3, if a direct translation exists for the old URLs into the new ones, just tell me about it and I will fix them :)