[Enhancement] Refactor main page readme. (#1609)

* add backtotop and move demos to back

* add intro image

* fix intro

* test table

* refact model zoo

* fix image size.

* fix model zoo.

* put text-to-image ahead

* add top id

* remove intro back to top

* whats new

* whats new update

* fix whats new

* major features.

* add demos

* fix demos.

* put whats new ahead

* fix image colorization

* pre commit

* remove getting started and demos.

* add migration.

* add contributors graph

* contributing

* major features.

* use video in intro.

* fix whats new

* change video

* change video sr and video interpolation intro

* update intro video

* add emoji

* fix backto top

* add emoji major feature

* fix failed link

* add top emoji back

* combine sngan and proj gan

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Co-authored-by: Yanhong Zeng <[email protected]>