Add howto about migrating a package to package spec v2

[jsoriano]

Add documentation about how to solve validation issues found with packages following the package spec v2.

Part of elastic/package-spec#399.
Fixes #963 (additional implemented validations should include their own docs in this document).

[mrodm]

LGTM
And those examples with possible errors will help for sure developers to upgrade the packages from v1 to v2 👍

[elasticmachine]

💔 Build Failed

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Build stats

• Start Time: 2022-09-08T11:40:36.676+0000

• Duration: 31 min 16 sec

Test stats 🧪

Test	Results
Failed	0
Passed	808
Skipped	0
Total	808

Steps errors

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authenticate

• Took 0 min 1 sec . View more details here
• Description: gcloud auth activate-service-account --key-file ${FILE_CREDENTIAL}

🤖 GitHub comments

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• /test : Re-trigger the build.

[mtojek]

@v1v It looks like the job is still being triggered by beats-ci?

https://beats-ci.elastic.co/blue/organizations/jenkins/Ingest-manager%2Felastic-package-package-storage-publish/detail/PR-964/2/pipeline

[mtojek]

nit: I would add a prefix "Troubleshooting" as this is rather information about how to fix an issue.

[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (33/33)	💚
Files	67.213% (82/122)	👍
Classes	62.941% (107/170)	👍
Methods	49.713% (346/696)	👎 -0.216
Lines	33.355% (3151/9447)	👍
Conditionals	100.0% (0/0)	💚

[jsoriano]

Failure is unrelated, merging.