Make `save_html()` an S3 generic method #699

Job	Run time
routine / routine	3m 13s
website / pkgdown	3m 57s
R-CMD-check / setup	22s
R-CMD-check / macOS-latest (4.3.1)	10m 1s
R-CMD-check / windows-latest (devel)	6m 18s
R-CMD-check / windows-latest (4.3.1)	11m 12s
R-CMD-check / windows-latest (4.1)	10m 16s
R-CMD-check / windows-latest (3.6)	8m 32s
R-CMD-check / ubuntu-20.04 (devel)	3m 22s
R-CMD-check / ubuntu-20.04 (4.3.1)	3m 18s
R-CMD-check / ubuntu-20.04 (4.2.3)	3m 30s
R-CMD-check / ubuntu-20.04 (4.1.3)	5m 6s
R-CMD-check / ubuntu-20.04 (4.0.5)	3m 52s
R-CMD-check / ubuntu-20.04 (3.6.3)	5m 1s
	1h 18m 0s

Provide feedback