doc: fix org change that link dead for discuss

[yihong0618]

fix #ISSUE_Number

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Change logs

As cloudberry the org changed to apache now, most of the links can redirect but for the discuss such as orgs/cloudberry/discuss can not, this PR fix that case.
you can try:
link dead: https://github.com/orgs/cloudberrydb/discussions/categories/proposal
link fixed: url: https://github.com/apache/cloudberrydb/discussions/categories/proposal

for the case it may graduate from apache some day so keep the urls can redirect first.

Why are the changes needed?

Describe why the changes are necessary.

Does this PR introduce any user-facing change?

If yes, please clarify the previous behavior and the change this PR proposes.

How was this patch tested?

Please detail how the changes were tested, including manual tests and any relevant unit or integration tests.

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[github-actions]

Hiiii, @yihong0618 welcome!🎊 Thanks for taking the effort to make our project better! 🙌 Keep making such awesome contributions!

[avamingli]

LGTM. Thanks.

[tuhaihe]

Hi @yihong0618, thank you! There is already a PR #696 for the link changes. Please feel free to rebase your commit once #696 is merged.

BTW, welcome to take the git commit template to amend your commit title and body. Thanks again!