docs: make buildx build the canonical reference doc

[dvdksn]

- What I did

Move common flag descriptions to the buildx build reference, and make that page the canonical page in docs. Also rewrite some content in image_build to make clear that this page is only for the legacy builder.

Relates to:

• https://github.com/docker/docs/pull/19774
• https://github.com/docker/buildx/pull/2397

- How I did it

- How to verify it

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 61.44%. Comparing base (0340921) to head (e91f0de).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5002      +/-   ##
==========================================
+ Coverage   61.42%   61.44%   +0.01%     
==========================================
  Files         298      298              
  Lines       20798    20805       +7     
==========================================
+ Hits        12776    12783       +7     
  Misses       7109     7109              
  Partials      913      913              

[dvdksn]

bump @thaJeztah - see also related:

• https://github.com/docker/docs/pull/19774
• https://github.com/docker/buildx/pull/2397

[dvdksn]

~I wanted to cherrypick to 27.0 but seems like a bit of a mess, so leaving it for 28. I should still be able to vendor docs from master.~

never mind, backport here: https://github.com/docker/cli/pull/5227