chore gitignore copyrighted Heavy Duty book material

doc/ holds the HD1 source used as reference for domain steering. It is copyrighted (1993 Mentzer-Sharkey Enterprises) and must never be committed. The steering docs may cite it by path, but the text itself stays local only.

Commit
1c9635c1ec8b12d493eff72e77401bf8890f9022
Author
Marius Peter <dev@marius-peter.com>
Author date
Committer
Marius Peter <dev@marius-peter.com>
Committer date
.gitignore
index 262064ea..d245303d 100644..100644
@@ -10,6 +10,9 @@
10 10 # Local Kiro tooling state (auto-generated, not project config)
11 11 .kiro/settings/
12 12
13 Added: # Copyrighted Heavy Duty book material — reference only, never commit
14 Added: doc/
15 Added:
13 16 # Editor / OS noise
14 17 *~
15 18 .#*