[hotfix][docs] add more information about how to speed up maven build.

[JingGe]

What is the purpose of the change

This pull request improves the information about how to speed up the maven build, which will help Flink developer for their daily work.

Brief change log

• Briefly describe the "fast" maven profile.
• Show alternative option of controlling skips manually.
• Recommend maven parallel build.

Verifying this change

This change is a document improvement without any test coverage.

Documentation

• Does this pull request introduce a new feature? (yes / no)

[flinkbot]

CI report:

• 2a955c6 Azure: SUCCESS

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Automated Checks

Last check on commit 4b0f477 (Tue Oct 12 10:47:12 UTC 2021)

Warnings:

• Documentation files were touched, but no docs/content.zh/ files: Update Chinese documentation or file Jira ticket.

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Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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