[site,doc] Ease documentation reading on large screen

@hcallahan-lowrisc is already working on this.

Since Jan 10, 2024.

### Description

Several pages cannot be easily read because the CSS definition limits the width of the content to an arbitrary width of 750 pixels. For example, `hw/ip/otp_ctrl/doc/theory_of_operation.html#partition-listing-and-description` is hardly readable because the table is always larger than the actual view.

I do not think it should be limited to a pixel size, as the results really depends on the reader's browser. Moreover whatever the viewport size, the content width is limited to 750px. I think it would be better (and far more readable) to let the reader pick the width he wants rather than locking him down.

Removing `--content-max-width: 750px;` from `site/book-theme/css/variables.css` improves a lot the read experience, but comes with its own drawback:
* right-side ToC may become a bit narrow; perhaps it could be defined with a min width based on element (`em`) rather than on pixels
* SVG diagrams may use the whole content width and become a bit oversize, it may be possible to clamp their width to a ratio of the actual elements (`em`) so that their appearance is kept in sync with the actual character size.

Thanks.