Update Docs to rename Custom HTTPJSON to Custom API

[kcreddy]

What does this PR do?

Rename Custom HTTPJSON integration to Custom API

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

• [ ]

How to test this PR locally

Related issues

• Closes [Custom API] Update Docs #4484

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-10-30T08:39:48.490+0000

• Duration: 17 min 9 sec

Test stats 🧪

Test	Results
Failed	0
Passed	7
Skipped	0
Total	7

🤖 GitHub comments

Expand to view the GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (0/0)	💚
Files	100.0% (0/0)	💚 2.703
Classes	100.0% (0/0)	💚 2.703
Methods	100.0% (3/3)	💚 9.956
Lines	100.0% (0/0)	💚 8.418
Conditionals	100.0% (0/0)	💚

[elasticmachine]

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