docs: tutorials->tutorial and rename flask page

[cjdcordeiro]

• [x] Have you signed the CLA?

---

• rename Tutorials to Tutorial

  Tutorial because there should really be just one tutorial. One learning experience, even if in multiple sections.

• rename the Flask tutorial .rst file in order to achieve a smaller URL

• the docs are also versioned and without language, so this PR also removes the /en/ from the links in the codebase

[tigarmo]

Did you already change the readthedocs versioning scheme? Because now eg https://documentation.ubuntu.com/rockcraft/en/stable/reference/rockcraft.yaml is a 404 and that breaks links printed by stable Rockcraft. edit: i'm reverting that rtd change for now

[cjdcordeiro]

@tigarmo thanks for reverting it.

[cjdcordeiro]

Which older versions have tutorial links in the codebase? This current version doesn't have any