docs: clarify `CELESTIA_HOME` (#3644) #1

Job	Run time
docker-txsim-build / prepare-env	5s
docker-security-build / prepare-env	4s
docker-security-build / logic-check	1s
docker-txsim-build / logic-check	1s
docker-txsim-build / docker-security	3m 38s
docker-security-build / docker-security	3m 40s
docker-txsim-build / docker-build (DockerHub; docker.io/celestiaorg/txsim)	5s
docker-txsim-build / docker-build (GHCR; ghcr.io/celestia-org/txsim)	8m 54s
docker-txsim-build / docker-build (ScaleWay; rg.fr-par.scw.cloud/celestiaorg/txsim)	8s
docker-security-build / docker-build (DockerHub; docker.io/celestiaorg/celestia-app)	5s
docker-security-build / docker-build (GHCR; ghcr.io/celestia-org/celestia-app)	3m 32s
docker-security-build / docker-build (ScaleWay; rg.fr-par.scw.cloud/celestiaorg/celestia-app)	8s
	20m 21s

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