Switch to absolute URLs in readmes so links aren't broken on PyPI.

[erikrose]

Also add a conditional quick-start step to install Flask or Bottle, because our examples fail at build time when those aren't around.

[posborne]

I think we should really avoid having users do pip install steps at all in favor of having dependencies be modeled in pyproject.toml for relevant examples and pushing uv; I glossed over the pip instructions on first read and think it is detrimental given our prior decision to make uv the paved path and the greater importance of having an isolated virtualenv or similar for targeting the wasm environment.

The manual steps listed are likely to go away in favor of templates for a better getting-started experience, but even then I think as-is, this is probably pushing users down a suboptimal path. Even though there's pretty good flexibility in terms of how things can be done, I think it may be better to document as if use of uv is what is to be used.