Add direct links to github source code to docs (#7738)

### Description

This adds direct links to the respective source code of classes,
methods, etc. to the docs.
In my opinion these nicer and more useful than viewing the source code
in copy in the docs.

![image](https://github.com/Project-MONAI/MONAI/assets/20091488/4a6a2650-9fd2-4cd9-a2a1-084cc3a5fb36)

I currently added it in addition to the links to the source files copied
into the docs. If desired this could also be done instead of.
I also used a github logo instead of another `[source]` link.
I am not sure whether there is any easier way to change that into the
logo than the way I have done, but this one seems to work.


### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [x] Documentation updated, tested `make html` command in the `docs/`
folder.

---------

Signed-off-by: John Zielke <j.l.zielke@gmail.com>

Author: johnzielke

docs/source/conf.py

@@ -13,6 +13,8 @@
 import os
 import subprocess
 import sys
+import importlib
+import inspect
 
 sys.path.insert(0, os.path.abspath(".."))
 sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), "..", "..")))
@@ -95,7 +97,7 @@ def generate_apidocs(*args):
     "sphinx.ext.mathjax",
     "sphinx.ext.napoleon",
     "sphinx.ext.autodoc",
-    "sphinx.ext.viewcode",
+    "sphinx.ext.linkcode",
     "sphinx.ext.autosectionlabel",
     "sphinx.ext.autosummary",
     "sphinx_autodoc_typehints",
@@ -162,3 +164,54 @@ def setup(app):
     # Hook to allow for automatic generation of API docs
     # before doc deployment begins.
     app.connect("builder-inited", generate_apidocs)
+
+
+# -- Linkcode configuration --------------------------------------------------
+DEFAULT_REF = "dev"
+if os.environ.get("GITHUB_REF_TYPE", "branch") == "tag":
+    # When building a tag, link to the tag itself
+    git_ref = os.environ.get("GITHUB_REF", DEFAULT_REF)
+else:
+    git_ref = os.environ.get("GITHUB_SHA", DEFAULT_REF)
+
+DEFAULT_REPOSITORY = "Project-MONAI/MONAI"
+repository = os.environ.get("GITHUB_REPOSITORY", DEFAULT_REPOSITORY)
+
+base_code_url = f"https://github.com/{repository}/blob/{git_ref}"
+MODULE_ROOT_FOLDER = "monai"
+
+
+# Adjusted from https://github.com/python-websockets/websockets/blob/main/docs/conf.py
+def linkcode_resolve(domain, info):
+    if domain != "py":
+        raise ValueError(
+            f"expected domain to be 'py', got {domain}."
+            "Please adjust linkcode_resolve to either handle this domain or ignore it."
+        )
+
+    mod = importlib.import_module(info["module"])
+    if "." in info["fullname"]:
+        objname, attrname = info["fullname"].split(".")
+        obj = getattr(mod, objname)
+        try:
+            # object is a method of a class
+            obj = getattr(obj, attrname)
+        except AttributeError:
+            # object is an attribute of a class
+            return None
+    else:
+        obj = getattr(mod, info["fullname"])
+
+    try:
+        file = inspect.getsourcefile(obj)
+        source, lineno = inspect.getsourcelines(obj)
+    except TypeError:
+        # e.g. object is a typing.Union
+        return None
+    file = os.path.relpath(file, os.path.abspath(".."))
+    if not file.startswith(MODULE_ROOT_FOLDER):
+        # e.g. object is a typing.NewType
+        return None
+    start, end = lineno, lineno + len(source) - 1
+    url = f"{base_code_url}/{file}#L{start}-L{end}"
+    return url