Independent Sphinx Stack config?

In the past, I have found updating to newer versions of the starter pack/Sphinx Stack to be cumbersome, because the `conf.py` file includes general stuff that's part of the Sphinx Stack, but it also contains project-specific stuff such as the project title, associated project links (GitHub, Discourse, etc). This means that updates to the Sphinx Stack, which may change extensions and other behaviors, have to be manually cherry-picked into the `conf.py` file so that you get the updates you want without overwriting the project-specific stuff.

Could the Sphinx Stack provide some default values, and then `conf.py` would just pull those values by default, but be able to override them otherwise? As a very rough picture, I envision something like

```python
# Somewhere in the Sphinx Stack code
SPHINX_STACK_EXTENSIONS = [...]
SPHINX_STACK_LINKCHECK_IGNORE = [...]
# etc, with other values

# In the project docs/conf.py
from sphinx_stack import SPHINX_STACK_EXTENSIONS, SPHINX_STACK_LINKCHECK_IGNORE

# The default would just set the local values to the Sphinx Stack values,
# but the project could extend or completely override them as desired
extensions = SPHINX_STACK_EXTENSIONS + [...]
linkcheck_ignore = SPHINX_STACK_LINKCHECK_IGNORE + [...]
```

Start by examining the project’s docs/conf.py and locating the Sphinx Stack configuration code that supplies shared extensions, linkcheck settings, and other defaults. Map which values are project-specific versus shared; done means projects can consume stack defaults while extending or overriding them without manually cherry-picking updates.

Written by the indexing model from the issue text.