ShadowRoot.serializable, getHTML() etc add MDN urls

[hamishwillee]

Adds MDN urls for Shadowroot methods Shadowroot.getHTML, Shadowroot.serializable, Element.getHTML(). Follows on from work done in #23364

Related docs work can be tracked in mdn/content#34302

[Elchi3]

Thanks for your PR, @hamishwillee but I automated this work away this week :)

Going forward, you don't need to worry about adding mdn_urls to BCD anymore. See the new linter in #23431 that will do this work several times a week.

These specific mdn_urls were already added in the initial run of the liner: #23481

[hamishwillee]

Thanks @Elchi3 . Sounds like an excellent and cunning plan.