improvement(cmake): simplify build option and doc

[tpoisonooo]

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Motivation

Build option is too complex for me. Now that I have specified ncnn_DIR=xx, why have I typing TARGET_BACKENDS=ncnn...

If you want to use ncnn, cmake -Dncnn_DIR=${NCNN_DIR}/build/install/lib/cmake/ncnn .. should be all you need.

Please first review https://github.com/open-mmlab/mmdeploy/pull/842

Modification

• Remove repeated MMDEPLOY_TARGET_BACKENDS option
• Remove redundant build option description in doc, add a cmake_option.md
• Using g++-7 if g++ compiler not specified
• Update doc

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has a dependency on downstream projects of a newer version, this PR should be tested with all supported versions of downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov[bot]]

Codecov Report

Merging #832 (9d640d0) into master (f80c90e) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 9d640d0 differs from pull request most recent head b345756. Consider uploading reports for the commit b345756 to get more accurate results

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Flag	Coverage Δ
unittests	54.65% <ø> (ø)

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[hanrui1sensetime]

May be still need change jetsons.md to remove -DMMDEPLOY_TARGET_BACKENDS options explicitly. The same about zh-cn version.

[AllentDan]

May update the building way of the dockerfile as well.

[tpoisonooo]

May update the building way of the dockerfile as well.

done.

[tpoisonooo]

Please fix CircleCI first, and then LGTM.

done.