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

[ShahanaFarooqui]

Excellent work on the markdown output!

To make it even more useful for release notes, may I suggest a couple of refinements:

• The list should mention the sorting pattern, like "(sorted by commit count, descending)". This gives the most relevant contributors top visibility and makes the ordering transparent to readers.
• Instead of separate sections, consolidate the list into a single section with a (*) marker for first-time contributors. This avoids redundancy while still highlighting newcomers.
• A single line at the bottom thanking first-time contributors keeps the recognition without cluttering the list.

This can provide us a clean, scannable list that still celebrates first-timers. For example:

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

### Contributors (ordered from most to fewest commits)

- 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

[sangbida]

Thanks @ShahanaFarooqui! It now produces an output like this:

sangbida@mac lightning % ./devtools/credit --markdown v26.04         
## Since **v26.04** we've had 303 commits in 70 days by 22 authors

### Contributors (ordered from most to fewest commits)

- Rusty Russell (128 commits)
- Sangbida Chaudhuri (44 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

[ShahanaFarooqui]

Thanks, looks good :).

Ack a087d78