Documentation: the package size limit includes the layers

[mnapoli]

In "Reducing package size":

• The 250MB limit counts the application code and the layers: Bref's PHP runtime layer takes about 80MB of it (e.g. the PHP 8.4 layer is ~79MB unzipped), which leaves roughly 170MB for the code. Without this, a 200MB application looks fine but can't be deployed.
• Quote the error AWS returns when a deployment is too large (Unzipped size must be smaller than 262144000 bytes), inline so that the docs search indexes it: pasting the error into the search now leads to this section.