[doc](readme) Fix missing Slack badge in localized README files

[yifanwu-sys]

What problem does this PR solve?

Some README files had an empty Slack anchor in the social media badge section:

<a href="https://doris.apache.org/slack" height=25></a>

Because the anchor did not contain an <img> element, the Slack badge
was not visible when the README was rendered.

What is changed?

This PR adds the missing Slack badge image to the affected README files,
following the existing correct implementation in docs/zh-CN/README.md
and keeping it consistent with the adjacent social media badges.

<a href="https://doris.apache.org/slack"><img src="https://img.shields.io/badge/-Slack-4A154B?style=social&logo=slack" height=25 alt="Slack"></a>

Check

• All affected README files now contain the Slack badge image.
• No README file still uses the empty Slack anchor.
• docs/zh-CN/README.md was left unchanged because it already had the
  correct badge.

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[yifanwu-sys]

Thanks for the reminder. I have clarified the PR description below.

What problem was fixed?

This PR fixes a documentation rendering inconsistency in the README social media badge section.

In some localized README files, the Slack link was written as an empty anchor:

<a href="https://doris.apache.org/slack" height=25></a>

Because the anchor did not contain an <img> element, the Slack badge was not visible when the README was rendered.

docs/zh-CN/README.md already had the correct implementation, so I used it as the reference.

How was it fixed?

I added the missing Slack badge image element to the affected README files:

<a href="https://doris.apache.org/slack"><img src="https://img.shields.io/badge/-Slack-4A154B?style=social&logo=slack" height=25 alt="Slack"></a>

The change keeps the Slack badge consistent with the existing Twitter and Medium badges in the same social media section.

What behavior was changed?

Before this PR:

• The Slack anchor existed, but it was empty.
• The Slack badge was invisible in the affected README files.

After this PR:

• The Slack badge is visible.
• Users can see and access the Slack community link from the rendered README.
• The localized README files are consistent with the already-correct docs/zh-CN/README.md.

Why was it modified?

The modification was made to fix the missing visual badge and keep the README social media section consistent across localized versions.

Possible impact

This is a documentation-only change.

It does not affect:

• Doris build behavior;
• runtime behavior;
• SQL behavior;
• storage behavior;
• query execution;
• compatibility.

New features

No new feature is added.

Refactoring

No code was refactored.

Optimization

No runtime or functional optimization was made. This PR only fixes the missing Slack badge image in README files.

Verification

I verified that:

• the affected README files now contain the Slack badge image;
• no README file still uses the empty Slack anchor;
• docs/zh-CN/README.md was not modified because it already had the correct implementation.