docs(vue-next): add register custom-component doc & use case

[gguoyu]

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]

Hi, @gguoyu. Thanks for your PR! 👏

🏷️ 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]

After a quick scan, I have approved workflow to run.

• SHA: e4578b8

🏷️ New commits in this PR would not be tested automatically until this pull request is reviewed by our collaborators.
🏷️ No need to worry about the status of merge_guard and [gh] pull request merge guard / merge_guard (pull_request_target) checks, once this pull request is met merge requirements, it will be automatically converted to successful status.

[hippy-service]

🎉 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: e4578b8