Option to produce paths and filenames that match one-to-one with the sources

[nthiery]

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Thanks to #670 and site.options.folders, it is now possible to produce a web site whose folder structure matches the folder structure of the source directory. However there remains some directory / file name rewriting:

• file and directory names are rewritten to lowercase
• accented characters are stripped: e.g. combinatoire-énumérative is rewritten to combinatoire-num-rative

Proposal

When site.options.folders is on (or possibly with an additional option), ensure a one-to-one correspondence for folders and file names between the sources and the produced site.

Motivation:

• Make it simpler for users and tooling to retrieve and edit the original source file.
• Ease transition from Jupyter-Book by preserving the site layout, and thereby URL pointing to specific resources in the site.

Things to cover

The following things are removed from the file names:

• [ ] Accented characters
• [ ] Any numbers
• [ ] Capitalization is removed (converted to lower case)