[docs] Fix typos and grammar in documentation

[orangeCatDeveloper]

Purpose

Fix typos, grammar, and copy-paste mistakes across the documentation. No linked issue (typo/documentation fix, per the contribution checklist exception).

Brief change log

• Correct spelling and grammar in many docs (e.g. to to retry, underling, not a absolute, drop exists partitions, subject-verb agreement).
• Fix content errors:
  • client.security.sasl.username was described as "The password to use ..." → username.
  • List ACL example called sys.drop_acl → sys.list_acl.
  • Reversed bucket-key wording: default bucket key derives from the primary key, not vice versa (options.md, pk-table.md).
  • SASL JAAS package namespace org.alibaba.fluss → com.alibaba.fluss in the 0.8 upgrade notes.
  • Azure filesystem entry linked to "manually install the OBS plugin" → Azure plugin.
  • Helm image.repository default fluss → apache/fluss (matches helm/values.yaml).

Tests

Docs-only change. Built the site locally (build_versioned_docs.sh + docusaurus build); the changed pages introduce no broken links.

API and Format

No API or storage format changes.

Documentation

Documentation-only fixes.