Fix share card

[jaipurandare]

As discussed with Chris, using logo as the share card.
Unable to test locally.

[surfaceowl]

@jaipurandare --- thanks for this; would you please make a few changes.

1. please fix the size and/or aspect ratio of the image used for the share preview? It looks too wide in the view I see. I was unable to see the actual image preview from some of the web-based tools out there, and was using Chrome, and installed the social web preview chrome extension https://chromewebstore.google.com/detail/social-share-preview/ggnikicjfklimmffbkhknndafpdlabib to get the image below:

2. remove "2025" from all of the share preview tags? This seems like something we might easily overlook to update every year, and when we post links in say LinkedIn, we're going to write "PyBay 2025" or "PyBay 2026" anyway. I think this will also fix the share card having the term 2025 repeated twice -- it currently reads "PyBay 2025 -Annual San Francisco Python Conference 2025" -- The interest would be to eliminate future work.

3. investigate and fix why some tags are not found in LinkedIn Post Inspector https://www.linkedin.com/post-inspector/I see the tags in the updated commit (e.g. PyBay/templates/layout.html) If possible, would be nice to fix / and add all missing tags.

4. Document the steps to update and test Share Cards in a new Readme -- will help future volunteer teams.

[surfaceowl]

+new graphic to better fix expected dimensions of social share card (1200x630) without cropping; updated share card language.