Fix broken links (#2082)

Most of these links are only broken when rendered out of tree, e.g. on
the OpenXLA website: https://openxla.org/stablehlo. For example, the
"More Examples" links lead to e.g.
https://openxla.org/stablehlo/tests/interpret/add.mlir, which is 404.

For links under `docs/` it is possible to use relative paths.

Also, add labels to images used in spec. This will materialize as the
"alt" text, which is used by screen readers.

docs/compatibility.md

@@ -3,7 +3,7 @@
 StableHLO is a backward compatible ML compute opset inspired by HLO/MHLO.
 This document explains the kind and the extent of the compatibility guarantees
 that StableHLO provides, based on the process established in
-[the compatibility RFC](../rfcs/20220912-compatibility.md).
+[the compatibility RFC](https://github.com/openxla/stablehlo/tree/main/rfcs/20220912-compatibility.md).
 
 ## Versions
 
@@ -126,8 +126,8 @@ for roundtrip examples of using the Python Serialization APIs.
 
 ## Tests
 
-We have a compatibility suite in [stablehlo/tests/vhlo](../stablehlo/tests/vhlo)
-that involves [a comprehensive compendium of StableHLO ops](../stablehlo/tests/vhlo/stablehlo_legalize_to_vhlo.mlir)
+We have a compatibility suite in [stablehlo/tests/vhlo](https://github.com/openxla/stablehlo/tree/main/stablehlo/tests/vhlo)
+that involves [a comprehensive compendium of StableHLO ops](https://github.com/openxla/stablehlo/tree/main/stablehlo/tests/vhlo/stablehlo_legalize_to_vhlo.mlir)
 serialized for all supported StableHLO versions. For every pull request, we are
 testing both backward and forward compatibility - i.e. that the suite can be
 deserialized targeting HEAD (backward compatibility), that the compendium

docs/reference_checklist.md

@@ -63,7 +63,7 @@ on those fronts alongside the interpreter implementation.
            an open issue reflecting that discrepancy.
 1. In [interpreter tests](https://github.com/openxla/stablehlo/tree/main/stablehlo/tests/interpret):
     1. Add a file called `<op_mnemonic>.mlir`.
-    1. Write tests following the [testing guidelines](https://github.com/openxla/stablehlo/blob/main/docs/reference.md#testing-guidelines).
+    1. Write tests following the [testing guidelines](reference.md#testing-guidelines).
 1. In the [testdata directory](https://github.com/openxla/stablehlo/tree/main/stablehlo/testdata):
     1. Run any disabled tests that are covered by the newly added operation.
     1. If the tests pass, enable them by converting `RUN-DISABLED` to `RUN`.
@@ -94,13 +94,13 @@ on those fronts alongside the interpreter implementation.
        in this file, following the same naming guidelines noted above.
     1. Move any shape inference tests from the [ops_stablehlo.mlir](https://github.com/openxla/stablehlo/blob/main/stablehlo/tests/ops_stablehlo.mlir)
        file into this file.
-1. In [spec.md](link):
+1. In [spec.md](spec.md):
     1. Add a link to `stablehlo/tests/interpret/<op_mnemonic>.mlir`
        to the "Examples" section
-       (e.g. [More Examples](https://github.com/openxla/stablehlo/blob/main/docs/spec.md#add)).
+       (e.g. [More Examples](spec.md#add)).
     1. Make sure the spec only has 1 example.
-    1. Make sure the spec example follows the [testing guidelines](https://github.com/openxla/stablehlo/blob/main/docs/reference.md#testing-guidelines).
+    1. Make sure the spec example follows the [testing guidelines](reference.md#testing-guidelines).
     1. Make sure the spec example test is interpretable.
     1. Make sure the spec example is the same as what is in the ODS.
-1. In [status.md](https://github.com/openxla/stablehlo/blob/main/docs/status.md):
+1. In [status.md](status.md):
     1. Update the "Interpreter" column to `yes`.