Correct url in migration doc and modify to main branch in doc urls

[zhangtbj]

Changes

1. When I reviewing the migration doc: https://github.com/tektoncd/pipeline/blob/main/docs/migrating-v1alpha1-to-v1beta1.md
   I found some of the links are wrong, like:

• #pipelineresource-equivalent-catalog-tasks (Correct is: https://github.com/tektoncd/pipeline/blob/main/docs/migrating-v1alpha1-to-v1beta1.md#replacing-pipelineresources-with-tasks)
• #image-resource (Correct is: https://github.com/tektoncd/pipeline/blob/main/docs/migrating-v1alpha1-to-v1beta1.md#replacing-an-image-resource)
• docs/tasks.md#resultes (Correct is: https://github.com/tektoncd/pipeline/blob/main/docs/tasks.md#emitting-results)

Correct them to the right link

2. Then I also found some of the Tekton links are still using master, so modify them togheter.

/kind docs

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• Includes tests (if functionality changed/added)
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Release Notes

Correct the links in migration doc and modify url to use main branch

[zhangtbj]

/kind documentation
/ok-to-test

[zhangtbj]

/retest

[zhangtbj]

/retest

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[pritidesai]

docs/ is not needed here, both the .md files are under docs/

[pritidesai]

thanks @zhangtbj for all the fixes, looks good to me except one link is still broken, will be great if you can fix it 🙏

[zhangtbj]

Sure thing, @pritidesai .

I have correct it. Thanks for the review!

[pritidesai]

thank you @zhangtbj
/lgtm