MINOR: Add links to projects on README and update parquet-compatibility to parquet-testing

[raulcd]

Rationale for this change

While reading the README file I found that parquet-compatibility seems to be a wrong component and should be parquet-testing.
I also think adding links can help navigation for users interested while reading the README file.

What changes are included in this PR?

• Update from parquet-compatibility to parquet-testing
• Add links to different projects

Do these changes have PoC implementations?

Not required

[raulcd]

I also found that we are linking to a Twitter Code of Conduct

We hold ourselves and the Parquet developer community to a code of conduct as described by Twitter OSS: https://github.com/twitter/code-of-conduct/blob/master/code-of-conduct.md.

The first link does not exist anymore and the second link is outdated. Should this potentially be updated to the ASF code of conduct as other projects like Arrow?
https://github.com/apache/arrow/blob/main/CODE_OF_CONDUCT.md

[wgtmac]

+1

Thanks!