Fix tables for Markdown conversion in List of default Accept values

[teoli2003]

These are large tables that are not really nice to consult.

• I removed the columns Comments and put the few comments below the tables. This shortens the width of a lot of them (but not all).
• I merged UA and its version in the first column, this makes the table more readable and removed the need for rowspan attributes that are not supported by MD tables.

The info in the table is quite outdated, but this will be fixed later; I wanted to keep this PR as technical as possible.

Bookkeeping: This is part of #7536.

[github-actions]

Preview URLs

• https://pr7819.content.dev.mdn.mozit.cloud/en-US/docs/Web/HTTP/Content_negotiation/List_of_default_Accept_values

Flaws

None! 🎉

External URLs

URL: /en-US/docs/Web/HTTP/Content_negotiation/List_of_default_Accept_values
Title: List of default Accept values
on GitHub

• 🚨 http://kb.mozillazine.org/Network.http.accept.default (1 time) (Note! This may be a new URL 👀)
• https://bugzilla.mozilla.org/show_bug.cgi?id=170789 (2 times) (Note! This may be a new URL 👀)
• https://bugzilla.mozilla.org/show_bug.cgi?id=489071 (1 time) (Note! This may be a new URL 👀)
• https://docs.microsoft.com/en-us/archive/blogs/ieinternals/ie-and-the-accept-header (4 times) (Note! This may be a new URL 👀)
• https://searchfox.org/mozilla-central/search?q=image.http.accept (1 time) (Note! This may be a new URL 👀)