docs: Fix import error.

```
Running Sphinx v1.8.5
loading translations [en]... done

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/mithro-vtr/envs/latest/lib/python3.7/site-packages/sphinx/registry.py", line 472, in load_extension
    mod = __import__(extname, None, None, ['setup'])
  File "/home/docs/checkouts/readthedocs.org/user_builds/mithro-vtr/checkouts/latest/doc/_exts/sdcdomain/__init__.py", line 1, in <module>
    from sdcdomain import setup
ImportError: cannot import name 'setup' from 'sdcdomain' (/home/docs/checkouts/readthedocs.org/user_builds/mithro-vtr/checkouts/latest/doc/_exts/sdcdomain/__init__.py)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/mithro-vtr/envs/latest/lib/python3.7/site-packages/sphinx/cmd/build.py", line 303, in build_main
    args.tags, args.verbosity, args.jobs, args.keep_going)
  File "/home/docs/checkouts/readthedocs.org/user_builds/mithro-vtr/envs/latest/lib/python3.7/site-packages/sphinx/application.py", line 228, in __init__
    self.setup_extension(extension)
  File "/home/docs/checkouts/readthedocs.org/user_builds/mithro-vtr/envs/latest/lib/python3.7/site-packages/sphinx/application.py", line 449, in setup_extension
    self.registry.load_extension(self, extname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/mithro-vtr/envs/latest/lib/python3.7/site-packages/sphinx/registry.py", line 475, in load_extension
    raise ExtensionError(__('Could not import extension %s') % extname, err)
sphinx.errors.ExtensionError: Could not import extension sdcdomain (exception: cannot import name 'setup' from 'sdcdomain' (/home/docs/checkouts/readthedocs.org/user_builds/mithro-vtr/checkouts/latest/doc/_exts/sdcdomain/__init__.py))

Extension error:
Could not import extension sdcdomain (exception: cannot import name 'setup' from 'sdcdomain' (/home/docs/checkouts/readthedocs.org/user_builds/mithro-vtr/checkouts/latest/doc/_exts/sdcdomain/__init__.py))
```

doc/_exts/archdomain/__init__.py

@@ -1 +1,33 @@
-from archdomain import setup
+from sphinxcontrib.domaintools import custom_domain
+from sphinx.util.docfields import *
+
+def setup(app):
+    app.add_domain(custom_domain('ArchDomain',
+        name  = 'arch',
+        label = "FPGA Architecture",
+
+        elements = dict(
+            tag = dict(
+                objname       = "Attribute",
+                indextemplate = "pair: %s; Tag Attribute",
+                fields        = [
+                    GroupedField('required_parameter',
+                        label = "Required Attributes",
+                        names = [ 'req_param' ]),
+                    GroupedField('optional_parameter',
+                        label = "Optional Attributes",
+                        names = [ 'opt_param' ]),
+                    Field('required',
+                        label = "Tag Required",
+                        names = [ 'required' ])
+                ]
+            ),
+            #var   = dict(
+                #objname = "Make Variable",
+                #indextemplate = "pair: %s; Make Variable"
+            #),
+            #expvar = dict(
+                #objname = "Make Variable, exported",
+                #indextemplate = "pair: %s; Make Variable, exported"
+            #)
+        )))

doc/_exts/rrgraphdomain/__init__.py

@@ -1 +1,25 @@
-from rrgraphdomain import setup
+from sphinxcontrib.domaintools import custom_domain
+from sphinx.util.docfields import *
+
+def setup(app):
+    app.add_domain(custom_domain('RRGraphDomain',
+        name  = 'rrgraph',
+        label = "Routing Resource Graph",
+
+        elements = dict(
+            tag = dict(
+                objname       = "Attribute",
+                indextemplate = "pair: %s; Tag Attribute",
+                fields        = [
+                    GroupedField('required_parameter',
+                        label = "Required Attributes",
+                        names = [ 'req_param' ]),
+                    GroupedField('optional_parameter',
+                        label = "Optional Attributes",
+                        names = [ 'opt_param' ]),
+                    Field('required',
+                        label = "Tag Required",
+                        names = [ 'required' ])
+                ]
+            ),
+        )))

doc/_exts/sdcdomain/__init__.py

@@ -1 +1,26 @@
-from sdcdomain import setup
+from sphinxcontrib.domaintools import custom_domain
+from sphinx.util.docfields import *
+
+def setup(app):
+    app.add_domain(custom_domain('SdcDomain',
+        name  = 'sdc',
+        label = "SDC",
+
+        elements = dict(
+            command = dict(
+                objname      = "SDC Command",
+                indextemplate = "pair: %s; SDC Command",
+            ),
+            option = dict(
+                objname      = "SDC Option",
+                indextemplate = "pair: %s; SDC Option",
+            ),
+            #var   = dict(
+                #objname = "Make Variable",
+                #indextemplate = "pair: %s; Make Variable"
+            #),
+            #expvar = dict(
+                #objname = "Make Variable, exported",
+                #indextemplate = "pair: %s; Make Variable, exported"
+            #)
+        )))