doc: add prerequisites section for building documentation

[javiertoledos]

Missing prerequisites may hinder documentation contributions as it may throw errors when running make doc (specially the Python version) or when trying to validate spellcheck or linting.

Personally I found I had those tools missing when trying to compile the doc first time and Sphinx uses a python notation that is available since 3.8 but the error thrown is cryptic if python version is older.

[stgraber]

Hello,

Before we can review this pull request, we need all the commits to include a suitable Signed-off-by line. You can see CONTRIBUTING.md for more information on that.

Thanks!

[stgraber]

.tmp/doc/contributing.md:80: MD004 Unordered list style
.tmp/doc/contributing.md:81: MD004 Unordered list style
.tmp/doc/contributing.md:82: MD004 Unordered list style
.tmp/doc/contributing.md:83: MD004 Unordered list style

[stgraber]

You should use - here for the list so it matches the style of the others in this file.

Rather than add things like mdl and venv to the wordlist, it's best to just put them in a code block.

Given those requirements are specific to the documentation checking, it's probably best to move them into the Automatic documentation checks section.

So something like:

GitHub runs automatic checks on the documentation to verify the spelling, the validity of links, correct formatting of the Markdown files, and the use of inclusive language.

You can (and should!) run these tests locally as well with the following commands:
- Check the spelling: `make doc-spellcheck`
- Check the validity of links: `make doc-linkcheck`
- Check the Markdown formatting: `make doc-lint`
- Check for inclusive language: `make doc-woke`

To run the above, you will need the following:
- Python 3.8 or higher
- The `venv` python package
- The `aspell` tool for spellchecking
- The `mdl` markdown lint tool

[javiertoledos]

done, updated doc with comments above.

[stgraber]

thanks!