docs: Update notes on Argo+Flux collaboration

[dholbach]

It's been a while since we wrote most of the docs for the GitOps Engine. In the meantime the Flux project has announced the plans to build Flux v2 on top of the GitOps Toolkit. This PR aims to update the docs to reflect this.

For everyone hasn't followed the news closely: This was discussed between project leads of both projects and we consider the time of collaboration as very fruitful: both Argo and Flux developed a new vision on this quest.

Right now this is a draft PR still, some other bits might need updating but I'd defer the decision to maintainers of the GitOps Engine.

What I did in the PR already:

• update main README
• add some more backstory in the FAQ

What I can see as TODO (potentially part of subsequent PRs though):

• update diagram in main README (not sure where the source of this is)
• update other FAQ items to explain the direction and positioning of the GitOps Engine
• quite a few other Flux mentions (might still be useful to keep in spec documents?)

[daniel@reef gitops-engine ]$ git grep -i flux | cut -d':' -f1 | uniq -c
     11 docs/faq.md
      4 specs/deployment-repo-update.md
      7 specs/design-bottom-up.md
     22 specs/design-top-down.md
      6 specs/design.md
      3 specs/image-update-monitoring.md
[daniel@reef gitops-engine ]$ 

CCing @alexmt @jessesuen @jaypipes @2opremio @hiddeco @squaremo @stefanprodan

[codecov-commenter]

Codecov Report

Merging #126 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   55.95%   56.01%   +0.06%     
==========================================
  Files          25       25              
  Lines        2688     2701      +13     
==========================================
+ Hits         1504     1513       +9     
- Misses       1043     1045       +2     
- Partials      141      143       +2     

Impacted Files	Coverage Δ
pkg/diff/diff.go	65.55% <0.00%> (+0.15%)	⬆️

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[dholbach]

I could file separate issues for the remaining bits too if you like.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

No Coverage information
2.6% Duplication

[alexmt]

Looks good. Thank you @dholbach !