docs: add section on creating a testnets from mainnet exports (backport #19475)

[mergify]

This is an automatic backport of pull request #19475 done by Mergify.
Cherry-pick of 89bba66 has failed:

On branch mergify/bp/release/v0.50.x/pr-19475
Your branch is up to date with 'origin/release/v0.50.x'.

You are currently cherry-picking commit 89bba6611.
  (fix conflicts and run "git cherry-pick --continue")
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	added by them:   docs/docs/build/building-apps/05-app-testnet.md

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To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[mergify]

Cherry-pick of 89bba66 has failed:

On branch mergify/bp/release/v0.50.x/pr-19475
Your branch is up to date with 'origin/release/v0.50.x'.

You are currently cherry-picking commit 89bba6611.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	added by them:   docs/docs/build/building-apps/05-app-testnet.md

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally