Update documents in Nov. 2020

[kmyk]

This issue is just for a memo.

• [x] https://github.com/online-judge-tools/oj/pull/842 Make our documentation at Read the Docs deprecated
• [x] https://github.com/online-judge-tools/oj/pull/843 Update the contents of documents
• [x] https://github.com/online-judge-tools/oj/pull/844 Replace the name "Stress testing" with "Random testing"
• [x] https://github.com/online-judge-tools/oj/pull/845 Translate docs/INSTALL.md to Japanese
• [x] https://github.com/online-judge-tools/oj/pull/846 Translate the "Missing features" section in docs/getting-started.md to English
• [ ] Clean up docs/ directory