feat: update logo images in README and index files

[germa89]

Description

As the title. Update the logo with a more standarized one and with higher resolution.

Thanks to @Revathyvenugopal162 for generating the logo for me. I just created the dark mode one.

Issue linked

Close #3919

Checklist

• I have visited and read Develop PyMAPDL.
• I have tested my changes locally
• I have added necessary documentation or updated existing documentation.
• I have followed the PyMAPDL coding style guidelines.
• I have added appropriate unit tests that also ensure the minimal coverage criteria.
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved to the PR if any.
• I have assigned this PR to myself.
• I have marked this PR as draft if it is not ready to be reviewed yet.
• I have made sure that the title of my PR follows Commit naming conventions (e.g. feat: adding new MAPDL command)

Summary by Sourcery

Enhancements:

• Replace dark-mode and light-mode logo URLs with the new standardized high-resolution logo filenames in README.md and index.rst.

[ansys-reviewer-bot]

Thanks for opening a Pull Request. If you want to perform a review write a comment saying:

@ansys-reviewer-bot review

[sourcery-ai]

Reviewer's Guide

Updated README to use new standardized, higher-resolution logos by replacing srcset and img src URLs for dark and light themes.

File-Level Changes

Change	Details	Files
Updated logo references for dark and light themes	Replaced dark-mode logo srcset URL with new file name Replaced light-mode logo srcset URL with new file name Updated default img src URL to point to the new light-mode logo	README.md

Possibly linked issues

• Cannot import name "_parsefull" #1: The PR updates the logo images to a higher resolution, directly addressing the issue's low resolution.

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[Copilot]

Pull Request Overview

This PR updates the logo images across the PyMAPDL documentation and README files to use higher resolution, more standardized versions. The changes ensure consistent branding with better quality logos throughout the project.

• Replace logo images in both light and dark variants
• Update image references in documentation and README files
• Use descriptive filenames that indicate size variants (large/small)

Reviewed Changes

Copilot reviewed 2 out of 8 changed files in this pull request and generated 1 comment.

File	Description
doc/source/index.rst	Updates logo image paths to use new high-resolution variants for documentation
README.md	Updates logo image paths to use new standardized logo variants for the repository front page

[sourcery-ai]

Hey @germa89 - I've reviewed your changes and they look great!

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.33%. Comparing base (f021ab2) to head (ba8459b).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4112      +/-   ##
==========================================
- Coverage   91.39%   91.33%   -0.06%     
==========================================
  Files         189      189              
  Lines       15667    15667              
==========================================
- Hits        14319    14310       -9     
- Misses       1348     1357       +9     

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[germa89]

@pyansys-ci-bot LGTM.

[pyansys-ci-bot]

✅ Approving this PR because germa89 said so in here 😬