Make mappings table match javadoc style and remove deprecated attributes

[altrisi]

This PR makes the Mappings Data table style match the rest of the javadoc, and also removes unused or deprecated attributes.

Preview:

Preview HTML: AliasedBlockItem.zip (made by hand and Find and Replace) (note: this preview HTML should be placed in an existing doc at net/minecraft/item in order to work) (note: this preview HTML also contains the click action from #9).

Uses the classes declared by javadoc itself in order to get those styles.

The removed elements are:

• All the align="center" attributes. It's deprecated since long time ago. If wanted, use style="text-align: center" instead (or add it to a stylesheet matching that class)
• The border="1" attribute. It's also deprecated
• The <colgroup> tag. It is unused in this case
• The class="literal" from the <code> elements. There are no styles nor scripted actions declared for that class.

Things that could be better: In order to match what javadoc does, the last <td> element (L84) should have the colLast class instead of colSecond for consistency, although both classes (at least currently) have the same styling applied.

Please test this by actually running the program, I've still not been able to generate mappings with it.

This conflicts with #9, since this PR changes the line where the name="copyable" exists. Simple to fix though.

[liach]

Interesting fact is that java 15 now uses method-summary field-summary summary-table than methodSummary fieldSummary for classes. I will make yarn building use java 15 and use these newer classes for forms instead.