Adds GitLab Pages example

[xfbs]

Adds a GitLab Pages example, and a general section for WASM code size specific to CSR applications.

I am not sure if the WASM code size things are well-placed there, but I felt like I should mention them here.
If not, feel free to move that section elsewhere.

[gbj]

Thanks for taking the time to make a PR.

Two things

1. There's already a whole chapter on optimizing WASM binary size. Maybe you could look it over, compare to what you've written here, and add anything that's missing to that chapter?
2. Adding GitLab Pages at the top of the list makes the diff for this very hard to read, because git interprets that as deleting all of the GitHub Pages content and replacing it, and then adding it back in below. Could you edit this so GitLab Pages is appended to the end of the list as a fourth option, rather than inserted at the top as the fourth option? That will make it much easier for me to review.

Thanks!