Add help page describing liftover utility

[ch-kr]

Alicia Byrne is working to update the ClinGen gnomAD use guidance and said it would be helpful to create a help page describing the liftover feature in the browser.

We have a previous changelog entry describing this feature, but I agree it would be helpful to have a help page that discusses how the v4 liftover works. For example, variants that are in v4 but aren't in v2 don't appear to include a liftover link (example variant), and changing the dataset to v2 shows this:

rather than bringing you to a region view (which is the liftover behavior for variants in v2 but not v3 per the changelog).

It would also be helpful to include a note on this page that we will not be releasing a GRCh37 liftover version of v4