[flang][docs] Update the top "doc" page

[awarzynski]

Summary

• Added a note about "Classic Flang"
• sorted entries in ToC

[PeteSteinfeld]

Just one small change requested.

Thanks for doing this!

flang/docs/index.md

6

I would call them "compilers" rather than "frontends".

[awarzynski]

Change "frontends" to "compilers"

[awarzynski]

Thanks for taking a look, Pete! I'll merge this today if there are no further comments.

[MarkMurrayARM]

Thanks for the update!

flang/docs/index.md

3–10

What is the URI for LLVM Flang?

9

Is there a branch or development tree that has this code somewhere? The Flang in the regular tree delegates the real compiling to gfortran.

[awarzynski]

Added a link to "LLVM Flang"

[awarzynski]

Thanks for taking a look, Mark!

flang/docs/index.md

3–10

Added!

9

Yes, the development is happening in parallel in llvm-project and f18-llvm-project. But the goal is to move entirely to llvm-project ASAP. I'm not really in control when "ASAP" will actually happen :) (hopefully soon!) There's a risk that anything that I add today becomes out-of-date very soon (even if the transition is not complete, the relation between the two is very dynamic). I am also a bit reluctant to refer people to LLVM forks in the official LLVM documentation.

As for the gfortran situation, I documented it here. It's a "temporary" solution, so not really a "front page" material :)

[MarkMurrayARM]

Thanks for the clarifications!

flang/docs/index.md

 # Welcome to Flang's documentation
 
-Flang is LLVM's Fortran frontend
+Flang is LLVM's Fortran frontend that can be found
+[here](https://github.com/llvm/llvm-project/tree/main/flang). It is often
+referred to as "LLVM Flang" to differentiate itself from ["Classic
+Flang"](https://github.com/flang-compiler/flang) - these are two separate and
+independent Fortran compilers. LLVM Flang is under active development. While it
+is capable of generating executables for a number of examples, some
+functionality is still missing. See [GettingInvolved](GettingInvolved) for tips
+on how to get in touch with us and to learn more about the current status.
 
 ```eval_rst
 .. toctree::
    :titlesonly:
 
    ReleaseNotes
 ```
 
 # Contributing to Flang
 
 ```eval_rst
 .. toctree::
    :titlesonly:
 
-   GettingInvolved
-   FortranForCProgrammers
-   C++style
    C++17
-   PullRequestChecklist
+   C++style
+   FortranForCProgrammers
+   GettingInvolved
    ImplementingASemanticCheck
+   PullRequestChecklist
 ```
 
 # Design Documents
 
 ```eval_rst
 .. toctree::
    :titlesonly:
 
-   Overview
-   Preprocessing
-   Parsing
-   LabelResolution
-   ModFiles
-   Semantics
-   OpenMP-semantics
+   ArrayComposition
+   BijectiveInternalNameUniquing
+   Calls
+   Character
    ControlFlowGraph
-   FortranIR
-   IORuntimeInternals
-   f2018-grammar.md
-   OpenMP-4.5-grammar.md
    Directives
+   DoConcurrent
    Extensions
+   FlangDriver
+   FortranIR
+   FortranLLVMTestSuite
+   IORuntimeInternals
    Intrinsics
+   LabelResolution
+   ModFiles
+   OpenMP-4.5-grammar.md
+   OpenMP-semantics
    OptionComparison
+   Overview
    ParserCombinators
+   Parsing
+   Preprocessing
    RuntimeDescriptor
    RuntimeTypeInfo
-   Calls
-   Character
-   ArrayComposition
-   BijectiveInternalNameUniquing
-   DoConcurrent
-   FortranLLVMTestSuite
+   Semantics
+   f2018-grammar.md
 ```
 
 # Indices and tables
 
 ```eval_rst
 * :ref:`genindex`
 * :ref:`modindex`
 * :ref:`search`
 ```