DOC: Update sponsors list

[reshamas]

Issue with current documentation:

The sponsors list here is outdated.
Should also have a section for "Past Sponsors".

https://github.com/pymc-devs/pymc/blob/main/README.rst

References

References pymc-devs/pymc.io#107 (comment)

Idea or request for content:

No response

cc: @purna135

[reshamas]

@purna135 This has been our past process for adding sponsors. Main logo and card go into the "brand" repo.
https://github.com/pymc-devs/pymcon_web_series/issues/264

• add sponsor logo to pymc-devs/brand (@reshamas) adding card for adia lab brand#28
• add sponsor logo to pymc-devs/pymc/README.rst (@purna135) Sponsor: add Adia Lab sponsor logo to README.rst #6939
• add sponsor logo to pymc-devs/pymc.io/welcome.md (@purna135) add Adia Lab sponsor logo to welcome.md pymc.io#94
• add sponsor logo to PyMCon website: pymcon.com(@purna135) Add Adia Lab logo pymcon_web_series_website#130

[purna135]

Added logo and card to the Brand repo here: pymc-devs/brand#30. I will update the README once the PR is merged and the image is available to use