docs: add a branching support information to contributing

[baywet]

Goals:

• explain which branches accept/require fix syncs and why
• explain the link with OAS and asp.net release cycles

support policies:

• 1.X: until Nov 2026
• 2.X: until Nov 2028 (assuming dotnet migrates to 3.X for net 11 - LTS 3 years from nov 2025)
• 3.X: until Nov 2028 at least (depending on previous assumption + dotnet releases - STS 2 years from Nov 2026)

kinds of contributions

• 1.X would require only security fixes, accept bugfixes
• 2.X would require bug and security fixes until Nov 2026, then only require security fixes, and accept bugfixes
• 3.X would be mainline until OAI 3.3 comes out and requires additional breaking changes