convert website source to Markdown, print message from installer about Cygwin paths

[johnyf]

These changes include:

• convert the TLAPS website sources from HTML to Markdown. The conversion from Markdown to HTML is now done using pandoc. A little HTML remains here and there inside the Markdown, to preserve some of the original formatting. The resulting website looks almost the same with before.

• move tutorial material from under the directory doc/web/ to the directory doc/doc/.

• add a Makefile for the website, to:

  • copy specific files from under doc/doc/ to under doc/web/content/
  • call a Python script that prepends HTML headers and appends HTML footers to Markdown sources, and then converts them to HTML files by calling pandoc

• add a message to the installer that informs about PATH updates that may be needed on Windows in order to call a tlapm that has been installed within Cygwin, from a Windows command-line environment

• use lowercase paths for website files, update the website links accordingly, and also the JavaScript script for generating HTML headers (specifically the webpage menus)

Please find more details about these changes in the commit messages.

[johnyf]

Thank you for reviewing the changes and building the website, I have updated the changes to correct the errors.

In the renaming of filepaths to lowercase, there were links in HTML that I had not renamed, and those caused the linking errors in the built website.

[damiendoligez]

I think you'll need to run a link checker on this thing.

[damiendoligez]

I don't think we want to change our website technology at this point, and the installer program was removed some time ago. Closing this PR without merging. Raise your hand if you disagree.