Remove dead html links from contributor documentation

The hmtml links **/superlu.c.html and **/cg.c.html are dead and
have been changed to link to the superlu.c and cg.c source files,
respectively.

Remove dead html links from contributor documentation

The hmtml links **/superlu.c.html and **/cg.c.html are dead and
have been changed to link to the superlu.c and cg.c source files,
respectively.

The files do not exist in the repository. See:

```shell
find -type f -name *cg.c.html
find -type f -name *superlu.c.html
```

Moreover, it does not appear these html files ever existed in the git history.
See:

```shell
git log --all -- **/superlu.c.html
git log --all -- **/cg.c.html
```

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Typos

Convert some of the rst files to md with ./rst2myst convert --config config.yaml *.rst

Need to manually fix

(petsc-developers-communication-channels)= to (petsc_developers_communication_channels)

Convert some of the rst files to md with ./rst2myst convert --config config.yaml *.rst

Need to manually fix

(petsc-developers-communication-channels)= to (petsc_developers_communication_channels)=

[include/petscerror.h](/include/petscerror.h.html) to <a href="PETSC_DOC_OUT_ROOT_PLACEHOLDER/include/petscerror.h.html">include/petscerror.h</a>

and friends

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Fix minor typos in the faq, manual, install and dev docs