doc: Fix libpython getting-started links

[nkgotcode]

Description

Fixes #7451.

This updates the links to the Python introduction and Jupyter introduction from the libpython getting-started page. The linked generated manual pages are one directory above the libpython page, so the links need to use ../.

Motivation and context

The current links resolve under manuals/libpython/, where those generated manual pages do not exist. The target pages exist at manuals/python_intro.html and manuals/jupyter_intro.html.

How has this been tested?

Validated the relative URL resolution and confirmed that both manual source files exist:

$ python3 - <<'PY'
from pathlib import Path
from urllib.parse import urljoin

base = 'https://grass.osgeo.org/grass-stable/manuals/libpython/'
checks = {
    '../python_intro.html': Path('doc/python_intro.md'),
    '../jupyter_intro.html': Path('doc/jupyter_intro.md'),
}
for href, source in checks.items():
    resolved = urljoin(base, href)
    print(f'{href} -> {resolved}')
    print(f'{source}: exists={source.exists()}')
    if not source.exists():
        raise SystemExit(f'missing source: {source}')
print('OK: libpython getting-started links resolve to existing manual sources')
PY
../python_intro.html -> https://grass.osgeo.org/grass-stable/manuals/python_intro.html
doc/python_intro.md: exists=True
../jupyter_intro.html -> https://grass.osgeo.org/grass-stable/manuals/jupyter_intro.html
doc/jupyter_intro.md: exists=True
OK: libpython getting-started links resolve to existing manual sources

Confirmed the published target pages are available:

$ curl -fsSI https://grass.osgeo.org/grass-stable/manuals/python_intro.html
HTTP/2 200

$ curl -fsSI https://grass.osgeo.org/grass-stable/manuals/jupyter_intro.html
HTTP/2 200

Checked the patch formatting:

$ git diff --check

The full Sphinx documentation build was not run locally because this checkout does not have the generated build configuration file required by make -C python/grass/docs checksphinx.

Screenshots (if appropriate)

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• N/A: New feature (non-breaking change which adds functionality)
• N/A: Breaking change (fix or feature that would cause existing
  functionality to not work as before)

Checklist

• PR title provides summary of the changes and starts with one of the
  pre-defined prefixes
• My code follows the code style
  of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• N/A: I have added tests to cover my changes.

[echoix]

Also see #7526