Move Compilation.md from Wiki

[AirQuick]

This pull request copies Compilation.md from the latest Wiki.

• Why this PR is necessary is demonstrated in Add x:variable to schema and improve compiler support for x:variable #475:
  • We need to propose changes to documentation and example files at once
  • And then we need to collaborate on the proposed changes
• I don't think this PR will affect user-friendliness of documentation. Compilation.md talks about the internal details. The other docs in the current Wiki focus more on the ordinary usage.
• I chose tutorial/under-the-food/ just because I couldn't find a better place among the existing directories.
• Compilation.md in this pull request is an exact copy of the latest Wiki.
  • We'll make further changes on it or add its companion files via other pull requests including Add x:variable to schema and improve compiler support for x:variable #475.

Further work

• Remove Compilation.md from Wiki repository and its sidebar.
• Somewhere in How XSpec works, put a hyperlink to https://github.com/xspec/xspec/tree/master/tutorial/under-the-hood/.

[galtm]

Looks fine to me. Thanks for taking care of this.

[cirulls]

It looks good to me and improves the documentation.

[AirQuick]

@cirulls @galtm Thanks, merged.

[AirQuick]

Remove Compilation.md from Wiki repository and its sidebar.

Done

[AirQuick]

Somewhere in How XSpec works, put a hyperlink

Done