chore(docs): remove remaining spectrum references

[taylorcjohnson]

Description

This PR attempts to remove remaining references to the Gatsby Spectrum community in documentation. Currently, it is separated into two commits:
Removing Spectrum from contributing and README files
Removing Spectrum from #100DaysOfGatsby blog series

I have two outstanding questions:
Is it appropriate for this PR to remove the Spectrum reference from already published blog posts (i.e., the second commit)?

If so, there are two additional blogs posts (not yet included in the PR) where Chris Biscardi introduces Gatsby themes and specifically discusses the gatsby-themes channel in Spectrum (not simply linking out to them) - could someone suggest how (or if) I should modify these? Thank you in advance!

1. https://github.com/gatsbyjs/gatsby/blob/9ec02711eb21b4b446c5792a99618e67959e439b/docs/blog/2018-11-11-introducing-gatsby-themes/index.md
2. https://github.com/gatsbyjs/gatsby/blob/bc82867cec6076fa83adef4de319e8e6d03beb75/docs/blog/2019-02-27-reactiflux-q-and-a/index.md

Documentation

Not applicable - this PR does not change/add any code that would require documentation

Related Issues

Fixes #26351

[AishaBlake]

Hey, @taylorcjohnson! Thanks for following up on this! I generally prefer not to mess with blog posts once they've been published but I'll tag @mgienow in here for the final word. Once she's made a call, one way or the other, I'll be happy to review this.

[taylorcjohnson]

Sounds great @AishaBlake - that completely makes sense. Looks like the build failed due to dependency issues anyway (not sure why exactly), so I may wait until the final verdict and start over with a fresh fork 🤷?

Apologies for the triple reviewer request! Didn't realize GitHub would automatically add those maintainer groups with you already in them! 🤦

[muescha]

personally i would expect that blogpost get updated with with small bug fixes or generic information changes like the spectrum link.

[AishaBlake]

personally i would expect that blogpost get updated with with small bug fixes or generic information changes like the spectrum link.

In this specific case, yes, I'd agree. Regardless, I'd like to run this by Michelle. @taylorcjohnson, I appreciate you breaking this up into the two commits. Either way, this should be a smooth change!

[LekoArts]

I talked to @mgienow and while doing that realized that the blog posts are stored in WordPress now anyways (the live ones), so changing the markdown files here only helps when people browse the repository on their own.

I'll file an internal ticket to update the posts in WordPress. I also replaced Spectrum with our AskGatsbyJS twitter account in both READMEs