assignments: update course links

* source/compiler_stages/tc_7/tc_7.rst,
* source/compiler_stages/tc_8/tc_8.rst,
* source/compiler_stages/tc_9/tc_9.rst,
* source/compiler_stages/tc_x/tc_x.rst,
* source/compiler_stages/tc_y/tc_y.rst: here.
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