feat: Rename confusing update strategies

[jaideepr97]

This PR fixes #179.
It renames latest strategy to newest-build and name strategy to alphabetical. If either of the old names is detected, it prints a warning message in the controller logs informing the user that the strategies have been renamed, and that support for them will be removed in future releases. Currently the old names are still honored.
It also updates the documentation references for these update strategies.

Steps to test:

1. Run Argo CD locally
2. Create application with some image
3. Generate argo-cd token
4. annotate application with either newest-build or alphabetical depending on your use case/availability of test images
5. Run image-updater
6. app should be out of sync, and updated image should appear once app is successfully sync'd.
7. Redeploy application and annotate with latest or name strategy
8. Run image-updater
9. Verify that logs contain warning message informing user to switch to new naming convention
10. app should be out of sync, and updated image should appear once app is successfully sync'd.

[codecov-commenter]

Codecov Report

Merging #456 (8dcf374) into master (f12a5ab) will increase coverage by 0.10%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master     #456      +/-   ##
==========================================
+ Coverage   65.60%   65.70%   +0.10%     
==========================================
  Files          21       21              
  Lines        2035     2041       +6     
==========================================
+ Hits         1335     1341       +6     
  Misses        571      571              
  Partials      129      129              

Impacted Files	Coverage Δ
pkg/tag/tag.go	82.53% <ø> (ø)
pkg/image/version.go	40.69% <11.11%> (ø)
pkg/image/options.go	82.60% <100.00%> (+0.67%)	⬆️
pkg/registry/registry.go	54.95% <100.00%> (ø)

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[jannfis]

LGTM! Thank you!