docs: add realm type (gnolang#2441)

This adds the `Realm` type definition to the `std` package reference
docs and links is appropriately.

---------

Co-authored-by: Leon Hudak <33522493+leohhhn@users.noreply.github.com>

docs/reference/stdlibs/std/chain.md

@@ -105,9 +105,8 @@ origPkgAddr := std.GetOrigPkgAddr()
 ```go
 func CurrentRealm() Realm
 ```
-Returns current Realm object.
+Returns current [Realm](realm.md) object.
 
-[//]: # (todo link to realm type explanation)
 #### Usage
 ```go
 currentRealm := std.CurrentRealm()
@@ -118,7 +117,7 @@ currentRealm := std.CurrentRealm()
 ```go
 func PrevRealm() Realm
 ```
-Returns the previous caller realm (can be code or user realm). If caller is a
+Returns the previous caller [realm](realm.md) (can be code or user realm). If caller is a
 user realm, `pkgpath` will be empty.
 
 #### Usage

docs/reference/stdlibs/std/realm.md

@@ -0,0 +1,41 @@
+---
+id: realm
+---
+
+# Realm
+Structure representing a realm in Gno. See concept page [here](../../../concepts/realms.md). 
+
+```go
+type Realm struct {
+    addr    Address
+    pkgPath string
+}
+
+func (r Realm) Addr() Address {...}
+func (r Realm) PkgPath() string {...}
+func (r Realm) IsUser() bool {...}
+```
+
+## Addr
+Returns the **Address** field of the realm it was called upon.
+
+#### Usage
+```go
+realmAddr := r.Addr() // eg. g1n2j0gdyv45aem9p0qsfk5d2gqjupv5z536na3d
+```
+---
+## PkgPath
+Returns the **string** package path of the realm it was called upon.
+
+#### Usage
+```go
+realmPath := r.PkgPath() // eg. gno.land/r/gnoland/blog
+```
+---
+## IsUser
+Checks if the realm it was called upon is a user realm.
+
+#### Usage
+```go
+if r.IsUser() {...}
+```