Add doc about how to build

[fzyzcjy]

Changes

Close https://github.com/fzyzcjy/flutter_rust_bridge/issues/2417

Checklist

• [ ] An issue to be fixed by this PR is listed above.
• [ ] New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• [ ] ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• [ ] If this PR adds/changes features, documentations (in the ./website folder) are updated.
• [ ] CI is passing. Please refer to this page to see how to solve a failed CI.

Remark for PR creator

• ./frb_internal --help shows utilities for development.
• If fzyzcjy does not reply for a few days, maybe he just did not see it, so please ping him.

[codecov[bot]]

Codecov Report

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

Project coverage is 98.93%. Comparing base (ae970bf) to head (35406b1). Report is 17 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2421 +/- ## ======================================= Coverage 98.93% 98.93% ======================================= Files 494 494 Lines 20396 20396 ======================================= Hits 20179 20179 Misses 217 217 ```

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