Modernize Multisite administration guidance - #517

Summary

• update outdated Multisite user, plugin, theme, upload, and terminology guidance
• clarify the requirements and risks involved in switching network types
• replace duplicated legacy rewrite examples with canonical Apache and Nginx handbook links

Validation

• npx --yes prettier@3.6.2 --parser markdown --prose-wrap always --print-width 180 --check multisite/administration.md
• git diff --check
• verified that the file contains no review markers, non-breaking spaces, trailing whitespace, or lines longer than 180 characters