Make `save_html()` an S3 generic method (#411) #703

Job	Run time
website / pkgdown	2m 58s
R-CMD-check / setup	25s
routine / routine	2m 57s
R-CMD-check / macOS-latest (4.3.1)	5m 34s
R-CMD-check / windows-latest (devel)	6m 7s
R-CMD-check / windows-latest (4.3.1)	10m 29s
R-CMD-check / windows-latest (4.1)	10m 50s
R-CMD-check / windows-latest (3.6)	9m 26s
R-CMD-check / ubuntu-20.04 (devel)	3m 45s
R-CMD-check / ubuntu-20.04 (4.3.1)	3m 9s
R-CMD-check / ubuntu-20.04 (4.2.3)	3m 20s
R-CMD-check / ubuntu-20.04 (4.1.3)	3m 18s
R-CMD-check / ubuntu-20.04 (4.0.5)	3m 31s
R-CMD-check / ubuntu-20.04 (3.6.3)	4m 27s
	1h 10m 16s

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