docs: add comprehensive backport workflow documentation

This commit adds detailed documentation for the automated backport
workflow and updates the contribution guidelines to reference it.

New documentation (docs/backport-workflow.md):
- Complete overview of the automated backport process
- Step-by-step usage instructions with examples
- Detailed explanation of workflow triggers and label format
- Technical details about workflow implementation
- Conflict resolution procedures and best practices
- Multiple backport scenarios and examples
- Comprehensive troubleshooting guide

Updated contribution guidelines (docs/code_contribution_guidelines.md):
- Replaced detailed backport instructions with brief overview
- Added reference to the new detailed documentation
- Keeps contribution guidelines focused and concise

The detailed documentation provides:
- How to use backport labels correctly
- What happens when labels are added before/after merge
- How the workflow validates branches and handles errors
- Step-by-step conflict resolution instructions
- Solutions for common problems and edge cases
- Examples of valid vs invalid label formats

This documentation ensures contributors and maintainers have clear
guidance on using the automated backport workflow effectively.

docs/code_contribution_guidelines.md

@@ -10,6 +10,7 @@
    1. [Code Review](#code-review)
    1. [Rework Code (if needed)](#rework-code-if-needed)
    1. [Acceptance](#acceptance)
+   1. [Backporting Changes](#backporting-changes)
    1. [Review Bot](#review-bot)
 7. [Contribution Standards](#contribution-standards)
    1. [Contribution Checklist](#contribution-checklist)
@@ -232,6 +233,23 @@ these signatures intact, we prefer using merge commits. PR proposers can use
 
 Rejoice as you will now be listed as a [contributor](https://github.com/lightningnetwork/lnd/graphs/contributors)!
 
+## Backporting Changes
+
+After a PR is merged to master, it may need to be backported to release branches
+(e.g., `v0.20.x-branch`) to include the fix or feature in upcoming patch releases.
+
+The project uses an **automated backport workflow** to simplify this process. Simply
+add a label like `backport-v0.20.x-branch` to your merged PR, and a GitHub Action
+will automatically create a backport PR for you.
+
+For complete documentation on the automated backport workflow, including:
+- How to use backport labels
+- Handling merge conflicts
+- Multiple backports
+- Troubleshooting
+
+See [backport-workflow.md](backport-workflow.md)
+
 ## Review Bot
 
 In order to keep the review flow going, Lightning Labs uses a bot to remind