Add initial deprecation notices

[yusefnapora]

This adds some warning text to a few sections of the main specs document to signal that we're aware of its shortcomings and are working on improving the spec. The odd formatting is me abusing github markdown's table syntax to draw some attention to the text.

So far I've just added a pretty generic notice to the intro and the sections that seem (to me) to be the most problematic: architecture, interfaces and properties.

There are other sections that need work as well, but those three need the most attention in the short term, IMO.

Questions:

• do we agree that these sections should be considered deprecated?
• what other parts of the spec should have "warning notices"?
• right now all the notices link to Improve outdated / misleading specs #156 - perhaps it would be better to make one issue per document to track ideas for improving each individually?
• what's the best way to prioritize improving the docs?

Another larger question is whether we should go big and focus on #110 vs rehabilitating the existing specs.