Fix a few broken links (#87098)

References to headings need to be preceded with a slash. Also,
references to headings on the same page do not need to contain the name
of the document (omitting the document name means if the name changes
the links will still be valid).

I double checked the links by building [the
website](https://github.com/llvm/mlir-www):

```shell
./mlir-www-helper.sh --install-docs ../llvm-project website
cd website && hugo serve
```