Add ligatures for <<, >>, <<=, and >>=

[wpkelso]

Currently, there are no ligatures included for the <<, >>, <<=, >>= operators. This becomes a particular problem in C/C++, where when all ligatures are enabled, the <<= and >>= operators become <+<= and >+>= where the second character in each operator is the geq or leq ligatures.

[github-actions[bot]]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 fabric: Docs

Check ID	Status
docs-make (fabric, doctest)	success	✅
docs-make (fabric, html)	success	✅

These checks are required after the changes to docs/source-fabric/advanced/compile.rst.

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