Fix `CONTRIBUTING.md`
(formatting, typos, grammar, etc.).

Add and fix `dev/tutorial.md`
(formatting, typos, grammar, etc.).

Remove duplicate of the tutorial that was in `README.md`.

Add and fix `dev/environment.md`
(formatting, typos, grammar, etc.).

Remove `dev/dev-env.rst`, it was only a redirect to wiki,
and we now have `dev/environment.md` instead.

Add `dev/sample-changer-changes-and-maintenance.md`
(no fixes, only added the title).

Update `pygments` dependency for access to new lexer `jsx`
(added in version `2.17`).

GitHub: closes https://github.com/mxcube/mxcubeweb/issues/1153