update ZScale ZIA readme

[ellis-elastic]

added link to Zscaler documentation and improved the wording of the other links

What does this PR do?

added link to Zscaler documentation and improved the wording of the other links

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• [ ]

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• Start Time: 2022-07-20T01:06:04.187+0000

• Duration: 25 min 6 sec

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Name	Metrics % (covered/total)	Diff
Packages	100.0% (5/5)	💚
Files	100.0% (5/5)	💚 2.982
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Methods	100.0% (66/66)	💚 10.844
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Conditionals	100.0% (0/0)	💚

[bmorelli25]

It looks like something might've gone wrong with your merge.

[2022-05-04T10:52:33.651Z] README.md is outdated. Rebuild the package with 'elastic-package build'

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[bmorelli25]

[2022-06-29T08:32:41.575Z] README.md is outdated. Rebuild the package with 'elastic-package build'