For every webpage one creates, also include within it (perhaps in a font the same color as the background) the string like "Document ReFindingKey vturoagj". Also include the ReFindingKey vturoagj in the file name of the webpage (or for index.html, the name of the enclosing directory). If the webpage moves, and all the links to it die, the page can re-found (hence re-finding-key) again by web searching for vturoagj. And people know the ReFindingKey because they know the file name of the old (dead) link.
I'd recommend 8 random characters of 'a' through 'z', enough for 200 billion unique keys. Collisions are not a problem because there is presumably a human in the loop who can distinguish among the collisions.
Update: search UUID