Although it looks like our documentation needs an update to reflect this ...
https://helpx.adobe.com/sign/help/adobe_sign_for_salesforce_developer_guide.html
... the Apex global class echosign_dev1.EchoSignApiService has the method below, at least in v20+ of Adobe Sign for Salesforce.
global static EchoSignApiService.AgreementViewUrl getViewAgreementUrl(Id agreementId)
You should be able to use this to get the view/manage page from Sign, given the agreementId, to get the AgreementViewUrl inner class which just has a viewUrl property. For security reasons, the generated agreement URL only has a temporary lifespan so it generates a REST-HTTPS call to get a fresh URL from Adobe Sign services.
I'll add a task to our backlog to update the helpx documentation to reflect this.
If you want to link to the signing experience rather than the view/manage page, you will need to have used the API to get the signing URL.
global static List<EchoSignApiService.SigningUrl> getSigningUrls(Id agreementId)
With their of these, you will need to have called sendDocument(Id agreementId) first, which starts some background processes such that getViewAgreementUrl() and getSigningUrls() may not be immediately available so you'll need a retry mechanism.
