devtools: add --markdown output to credit for release notes

[sangbida]

Generate a comprehensive contributor list ordered by commits, with a separate section for first-time contributors like this:

## Since **v26.04** we've had 302 commits in 68 days by 22 authors

### Contributors

- Rusty Russell (128 commits)
- Sangbida Chaudhuri (43 commits)
- @daywalker90 (29 commits)
- Níckolas Goline (22 commits)
- Lagrang3 (21 commits)
- daywalker90 (12 commits)
- Vincenzo Palazzo (8 commits)
- @enaples (7 commits)
- Madeline (5 commits)
- Dusty Daemon (5 commits)
- Chandra Pratap (5 commits)
- Christian Decker (4 commits)
- wqxoxo (2 commits)
- ShahanaFarooqui (2 commits)
- Aditya Sharma (2 commits)
- lightningd (1 commits)
- ScuttoZ (1 commits)
- @sangbida (1 commits)
- Mattia (1 commits)
- MSimeZ (1 commits)
- @daeho-ro (1 commits)
- Andezion (1 commits)

### Special thanks to our first-time contributors

- Níckolas Goline (22 commits)
- lightningd (1 commits)
- Mattia (1 commits)
- MSimeZ (1 commits)
- @daeho-ro (1 commits)
- Andezion (1 commits)

Changelog-None