docs: convert cast build instructions from html to markdown
docs: convert cast build instructions from html to markdown

This patch converts Cast Build Instructions from
https://www.chromium.org/developers/how-tos/build-instructions-cast
to Markdown and check it in docs/ directory.

The process for doing this was done partially by command line tools
and partially manually after inspection at http://dillinger.io/.

The command lines to generate the markdown file were:

$ wget https://www.chromium.org/developers/how-tos/build-instructions-cast
$ pandoc -s -r html build-instructions-cast -o castbuild_instructions.md

BUG=None
TEST=opened cast_build_instructions.md in dillinger.io and visually
inspected it.
R=slan@chromium.org

Review URL: https://codereview.chromium.org/1811983004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#381943}

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