docs(ios): add iOS 3.0 upgrade guide

[wwwcg]

Before submitting a new pull request, please make sure:

• [ ] Test cases have been added/updated/passed for the code you will submit.
• [ ] Documentation has added or updated.
• [ ] Commit message is following the Convention Commit guideline with maximum 72 characters.
• [ ] Squash the repeat code commits, short patches are welcome.

[hippy-service[bot]]

Hi, @wwwcg. Thanks for your PR! :clap:

:label: You can leave a comment in this PR with #help tag when you need help (e.g. some status checks run failed due to internal issue), admin team members will help asap.

[hippy-service[bot]]

:tada: It seems that this pull request has been approved by all required reviewers. As it only contains one normal commit, I will rebase and merge it automatically via add action(rebase-merge) label.

• SHA: ea7b8a1a2e9ec687628ee617289804d5a9d0f683

[hippy-actions[bot]]

:warning: rebase-merge action execution failed, because of auto rebase and merge failed.

Please check the following details:

GraphQL: Can't disable auto-merge for this pull request. (disablePullRequestAutoMerge)

#help