docs: fix build error from libvips + sharp

[christianbundy]

What / Why

Problem: Sharp doesn't play nicely with a globally-installed version of
libvips, and throws an error when you try to generate the documentation.
with npm run build:static. This has been documented as an issue in the
Gatsby issue tracker: gatsbyjs/gatsby#22303

Solution: Update the dependencies to pull in a new version of Sharp
which is immune to this problem. I ran npm update, which is supposed
to modify package.json, but it only changed package-lock.json. I
removed node_modules and ran npm install and can confirm that his
resolves the problem.

References

• Related to: default starter throws sharp errors gatsbyjs/gatsby#22303

[ruyadorno]

Hi @christianbundy, thanks for taking the time to contribute ❤️ We appreciate it!

Unfortunately with our focus switching to npm7, changes to the current npm6 release line are going to be limited to critical fixes for security issues and major bugs only.

I'm really curious if this fix really applies to our docs since we haven't been noticing any problems in the output files or the build currently. Let me know if that's the case then we can follow up with it either in v6 or v7 depending on the severity of the possible problem.

All the best and thanks again! 😄