Clarify which doc version to fix

[t-kikuc]

What would you like to be added:

• Redefine and clarify the rule of contributing to docs, focusing on which doc version to fix.
  cf. Our current rule: https://github.com/pipe-cd/pipecd/tree/master/docs

• [optional] If possible, it would be better to inform contributors before opening PRs.
  (e.g. Auto check in CI? PullRequest template?)

Why is this needed:

• Newer contributors often make mistakes of which version to fix (dev, v0.N.x(latest), and older)
• It takes time for both contributors and reviewers to fix it.

[t-kikuc]

pipecd/docs/README.md

Lines 15 to 21 in 997339c

	Here are the flow of docs contribution regard some known scenarios:
	1. Update docs that are related to a specified version (which is not the latest released version):
	In such case, update the docs under `/docs-v0.x.x` is enough.
	2. Update docs for not yet released features or changes:
	In such case, update the docs under `/docs-dev` is enough.
	3. Update docs that are related to the latest released docs version:
	Change the docs' content that fixes the issue under `/docs-dev` and `/docs-v0.x.x`, they share the same file structure so you should find the right files in both directories.

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