examples: fix sort order of the first few examples

[oliver-sanders]

• The examples are documented in directory sort order.
• This adds numbers to the start of the simple examples to make them float to the top of the list.
• The remainder will be sorted alphabetically.

Remember to make clean if you want the change to take effect before rebuild.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.