[docs] move builders in to individual pages

[tejal29]

Relates to #2904

The builder page was getting super long. With the addition of maturity snippet on the RHS, the index needed scrolling
Now they are broken in 3 parts.

1. Local
2. Remote
3. Custom Build ( i created a new page for this since Custom Build had too many sub headings and if combined in Local, the index on the RHS again needed scrolling)

Other changes

1. Updated the links on the main build/_index.md to point to the sub pages.
   Each subpage has a link to the tool which is linked along with some descriptions.

[container-tools-bot]

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[balopat]

This will be awesome!

However, the breakdown should be local, on-cluster and Google Cloud Build, not local / remote / custom.

We support custom artifacts locally as well.
Thus:

• local => dockerfile, jib, bazel, custom
• cluster => dockerfile, custom
• gcb => dockerfile, jib

[codecov]

Codecov Report

Merging #3193 into master will not change coverage.
The diff coverage is n/a.

addressed comments

[container-tools-bot]

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[container-tools-bot]

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[container-tools-bot]

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