document `pre_run_shell_cmd_hook` and `post_run_shell_cmd_hook`

[boegel]

markdownlint isn't happy, and I don't understand why, the links to the subsections work fine...

markdownlint-cli2 v0.7.1 (markdownlint v0.28.2)
Finding: **/*.md !docs/demos !docs/version-specific
Linting: 63 file(s)
Summary: 4 error(s)
Error: docs/hooks.md:[12](https://github.com/easybuilders/easybuild-docs/actions/runs/6046813459/job/16409066224?pr=210#step:3:13)8:3 MD051/link-fragments Link fragments should be valid [Context: "[Hook arguments](#hooks-arguments)"] https://github.com/DavidAnson/markdownlint/blob/v0.28.2/doc/md051.md
Error: docs/hooks.md:129:3 MD051/link-fragments Link fragments should be valid [Context: "[Return value of hooks](#hooks-return-value)"] https://github.com/DavidAnson/markdownlint/blob/v0.28.2/doc/md051.md
Error: docs/hooks.md:[13](https://github.com/easybuilders/easybuild-docs/actions/runs/6046813459/job/16409066224?pr=210#step:3:14)0:3 MD051/link-fragments Link fragments should be valid [Context: "[Parse hook specifics](#parse-hook-specifics)"] https://github.com/DavidAnson/markdownlint/blob/v0.28.2/doc/md051.md
Error: docs/hooks.md:277:41 MD051/link-fragments Link fragments should be valid [Context: "[Parse hook specifics](#parse-hook-specifics)"] https://github.com/DavidAnson/markdownlint/blob/v0.28.2/doc/md051.md
Error: Failed with exit code: 1

I'll disable rule MD051 to make it happy...