Nextdoor is a legitimate company. There is a page about them here on Wikipedia [en.wikipedia.org
Its registration process is onerous: They require:
first line of address,
mobile phone number
For first line of street address, they wish to confirm it via either a phone call to your mobile or a postcard sent to your address. There is a note saying "We'll automatically mail you a verification postcard if we can't confirm your address another way." I wonder if someone might have created an account and given your address and you have just received said postcard after a while.