docs: fill 14 empty placeholder files with starter content, add missing SQL links to README

[CoderLiLe]

Summary

• Fill 14 empty (0-byte) placeholder *基础.md files with structured starter content (MySQL, Redis, MongoDB, ES, ClickHouse, PostgreSQL, Sentinel, Nacos, MyBatis, Netty, Gateway, Seata, SkyWalking, OAuth2)
• Add 4 missing LeetCode SQL practice links to README under MySQL section
• Remove a misplaced duplicate content file in JUC assets directory
• Delete all .DS_Store files from the project

Test plan

• All 14 previously empty files now contain content
• No broken links in README
• No empty .md files remain
• No .DS_Store files in project