Update Spectral Theory and Eigenvalues and Eigenvector Lectures (#175)

* update eigen

* change the graph format

* Fix rounding error

* finalize example

* update eigenII

* finalize the proof section

* correct reference

* update titles for consistency

* Apply suggestions from code review

Co-authored-by: zhc <[email protected]>

* style and small content change

* Fix minor problems

* add index

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Co-authored-by: HengCheng <[email protected]>
Co-authored-by: zhc <[email protected]>

Author: HumphreyYang