feat(site): improve performance by replacing page content

[fshovchko]

No description provided.

[ala-n]

Just a small note:
If we see that the service worker is going to give us a benefit -
for the result version, it should use the existing code environment (TS + webpack).
You don't need to make a separate folder and use a plain JS single file, and it's still a client-side code.

[codeclimate]

Code Climate has analyzed commit 41e552d and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 53.0% (2.3% change).

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