sagemathgh-39264: add parameter immutable to generators in `sage/grap…

…hs/digraph_generators.py` (part 1)

    
We add parameter `immutable` to some generators in
`age/graphs/digraph_generators.py`.

This is straightforward for most of them, except for `RandomTournament`
where we use `getrandbits` rather than repeated calls to `random()`.
This is similar to what is done in method
`Graph(...).random_orientation()`.

This PR is motivated by discussions in sagemath#39177.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [x] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#39264
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton