The saying is not a famous quote.
Precocious young Lisa is trying to sound smart by using sophisticated words like "doth" and "celestial" in a rhyme. Ever the intelligent overachiever, she's essentially trying to come up with a wisely worded quote of her own. But "hero strong and brave" is actually quite a cheesy, uninspired bit of phrase-making. And the use of "pave" to describe laying a pathway is a clear reach for something to rhyme with brave.
Before you take a second to think about it, the quote sounds impressive and potentially historical -- but it definitely isn't. That's the joke.