rfctr(html): promote HTMLDoc candidate methods (#3177)
**Summary**
Make `._find_articles()` and `._find_main` into `._articles` and
`._main` properties on HTMLDocument, respectively.
**Additional Context**
After prior refactorings, these two functions now each require only
`self` and can become `@lazyproperty`s on `HTMLDocument`. This ensures
they are computed at most once. In addition, their close relationship to
`HTMLDocument` is indicated by their membership as methods rather than
"loose" functions.
