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]

move down in the dump to be after the attributes

[gregw]

Rather than override dump and have to repeat the stuff from the base class, why don't we just add all these Dumpable collections as beans the the context and let the normal dump mechanism dump them?

[gregw]

Or better yet, add one Dumpable webapp bean, that dumps all these extra details

[lachlan-roberts]

I opened a separate issue for this (see #12655).
As its outside the scope of this documentation 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.