[doc] Recommend 3 FOLLOWER FEs in metadata best practices

[ixzc]

Versions

• dev
• 4.x
• 3.x
• 2.1 or older (not covered by version/language sync gate)

Languages

• Chinese
• English

Docs Checklist

• Checked by AI
• Test Cases Built
• Updated required version and language counterparts, or explained why not
• If only one language changed, confirmed whether source/translation counterparts need sync

Summary

• The metadata operation page still recommended deploying only one FOLLOWER-type FE as MASTER and making the rest OBSERVERs. That advice is outdated and conflicts with current deployment docs, which recommend 3 FOLLOWERs for production HA.
• Update Best Practices and the "FOLLOWER FE hangs up one after another" FAQ for current/dev, 4.x, and 3.x (English and Chinese). Majority-write behavior is kept; the conclusion is now to deploy 3 FOLLOWERs and avoid stopping a majority of them.
• 2.1 and older docs are unchanged.

Test plan

• Review the Best Practices and FAQ fix typo:'一下' -> '以下' #5 wording in current, 4.x, and 3.x English/Chinese pages
• Confirm the rendered docs after merge: /docs/dev/admin-manual/trouble-shooting/metadata-operation#best-practices