ANN: typescript, slate, bugfixes, better landing page content

[williamstein]

In the last day we:

• updated to Typescript 4.9.5
• updated to the latest version of https://www.slatejs.org, which powers our rich markdown editor, and also fixed a copy/paste issue with slate.
• fix some issues with zoom and table of contents and pages in the computational whiteboard
• greatly improved the SageMath landing page https://cocalc.com/features/sage
• and added a new video intro to the main https://cocalc.com landing page.

I'm planning to post a lot of "Announcements" like this summarizing what is happening with cocalc in this section of Discussions.