Add documentation for form limits & improve configuration via context attributes

[lachlan-roberts]

• Add documentation page for limiting form content.
• FormFields.from already reads this configuration from context attributes, but this PR makes it so that those context attributes delegate to setting the equivalent fields on ContextHandler.
• Add information about maxFormKeys and maxFormContentSize to the dump.

replaces #12232

[sbordet]

Please add a similar security directory in the operations guide.

There, make a similar section titled "Limiting Form Content" (same title), that points to that of the programming guide for example:

Forms can be a vector for denial-of-service attacks, like explained in xref:...[this section].

Then proceed to explain operation-guide specific configuration.

We should have these form limits as a Jetty module properties, and if we don't already, we should add them.

[gregw]

I think we could simplify the dump by just adding a bean to dump the extra info from a WebApp, rather than override dump again and repeat.

Perhaps a bit outside the scope of this PR?

[gregw]

Can you make it clear that the attributes apply to both core and webapp contexts. For core applications you should reference FormFields#onFielda and that the limits can be passed in there, else they are taken from context or server attributes.

addressed review