auto-generate docs for Flex

[yecol]

Currently the reference doc files for Flex is pre-built and hosted in the code-repo,
with path: /flex/docs

Please integrate the generate cmd to make docs, following the manner we used for the project:

• auto-gen the docs;
• please note that the generated files should be in gh-pages branch, rather than main;
• maybe output the generated files to a subfolder Flex under section REFERENCES.

[ds-ssj]

In this issue, do we need to generate html files directly under REFERENCES (similar to the GAE JAVA API) or do we need to re-describe them in the style of rst files (similar to the GAE API and Python API) and then generate html files? @yecol

[yecol]

The first solution is fine, no need to re-describe them. :)