[amber-docs] RFR: Adding first FAQ, per discussion in various locations lol

[David Alayachew]

I'm not married to the format. The hard work, of finding the discussions and linking them inline, has been done. Just want to make sure that we get SOMETHING out there, so that we can have a definitive place to link new folks too when they have questions, for further reading.

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Commit messages:
 - Update _index.md
 - Create simple_source_files_and_instance_main_methods.md

Changes: https://git.openjdk.org/amber-docs/pull/27/files
  Webrev: https://webrevs.openjdk.org/?repo=amber-docs&pr=27&range=00
  Stats: 14 lines in 2 files changed: 14 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/amber-docs/pull/27.diff
  Fetch: git fetch https://git.openjdk.org/amber-docs.git pull/27/head:pull/27

PR: https://git.openjdk.org/amber-docs/pull/27