Sphinx upgrade (#1581)

**Title:**
Upgrade Sphinx and Jinja2 versions.

**Summary:**

- Replaces PR #1579. The RC branch builds will be deferred to a later
date.
- Upgrade Sphinx to version 8.1 and Jinja2 to 3.1.6
- All demos in the `master` branch successfully build.
- Some reference formatting inconsistencies spotted.

**Notes**
 
Testing deployment of the built demos show inconsistencies in the way
references are formatted in the text and at the bottom of the demo.

Current Production website version:
<img width="821" height="117" alt="Screenshot 2025-10-24 at 3 35 35 PM"
src="https://github.com/user-attachments/assets/c44e8908-c71d-48ce-aa92-9ff947c14200"
/>
<img width="836" height="400" alt="Screenshot 2025-10-24 at 3 36 08 PM"
src="https://github.com/user-attachments/assets/d739d2af-003f-4845-b6fd-0cebb4e3021a"
/>

With the Sphinx upgrade:
<img width="832" height="117" alt="Screenshot 2025-10-24 at 3 35 23 PM"
src="https://github.com/user-attachments/assets/b4b63e77-77e2-4d45-b896-5833e10761a2"
/>
<img width="836" height="477" alt="Screenshot 2025-10-24 at 3 35 59 PM"
src="https://github.com/user-attachments/assets/e3740780-3cfc-4e9d-b238-78554d10cfa7"
/>

**Note**: The above rendering issues are fixed in [this
PR](XanaduAI/pennylane.ai-react#2368). We'll
merge this first so we can build and upload all the demos, then merge
the pennylane.ai PR to fix the rendering on the website.

---------

Co-authored-by: runora95 <runor.agbaire@xanadu.ai>

Author: doctorperceptron

conf.py

@@ -44,7 +44,7 @@
 
 # If your documentation needs a minimal Sphinx version, state it here.
 #
-needs_sphinx = "1.8.5"
+needs_sphinx = "8.1"
 
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom

dependencies/requirements-build.txt

@@ -1,11 +1,11 @@
 # Build dependencies
-sphinx>=5.0.2
+sphinx==8.1
 sphinx_gallery==0.17.1
-Jinja2==3.0.3
+Jinja2==3.1.6
 markupsafe==2.1.1
 pyyaml~=6.0.1
 pypandoc==1.5
-pennylane-sphinx-theme @ git+https://github.com/PennyLaneAI/pennylane-sphinx-theme.git@sphinx-update
+pennylane-sphinx-theme 
 uv~=0.7.18
 numpy~=1.24
 pennylane

poetry.lock

@@ -62,12 +62,12 @@ build-backend = "poetry.core.masonry.api"
 # Base dependencies needed to build the website without any code execution (*-norun)
 # When updating these, ensure that the accompanying /dependencies/requirements-build.txt is also updated.
 [tool.poetry.group.base.dependencies]
-sphinx = ">=5.0.2"
+sphinx = "8.1"
 sphinx_gallery = "0.17.1"
-Jinja2 = "3.0.3"
+Jinja2 = "3.1.6"
 markupsafe = "2.1.1"
 pyyaml = "^6.0.1"
-pennylane-sphinx-theme = { git = "https://github.com/PennyLaneAI/pennylane-sphinx-theme.git", branch = "sphinx-update" }
+pennylane-sphinx-theme = "0.25.0"
 pypandoc = "1.5"
 pennylane = "0.43.0"
 uv = "^0.7.18"