Rewrite README to use ./check and ./fix

[Eric-Arellano]

Part of #2800.

We don't want contributors to have to know what the different checks are. They should simply run ./check and ./fix and we handle it all for them. If something breaks, we should say in the error message how to fix things.

Adds instructions to spellcheck

We now tell you how to fix the problem, whereas before you had to read the README.

MDX:

❯ npm run check:spelling

> [email protected] check:spelling
> tsx scripts/js/commands/checkSpelling.ts

docs/guides/index.mdx:6:19 - Unknown word (Qiskitttt)
CSpell: Files checked: 91, Issues found: 1 in 1 file.


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🙅 There are unrecognized words (see above). If the word is a not a typo, there are two ways to allowlist it:

1. Add a comment on a dedicated line near the top of the MDX file, between the page metadata and the H1 heading, such as: {/* cspell:ignore hellllooooo, ayyyyy */}

2. If you expect this word to appear in other files, add it to a dictionary. Add names to the `scripts/config/cspell/dictionaries/people.txt` file. Add scientific or quantum specific words to the `scripts/config/cspell/dictionaries/qiskit.txt` file. If it doesn't fit in either category, add it to the `words` section in `scripts/config/cspell/cSpell. json`. The word is not case-sensitive.

Jupyter:

❯ npm run check:spelling

> [email protected] check:spelling
> tsx scripts/js/commands/checkSpelling.ts

CSpell: Files checked: 91, Issues found: 0 in 0 files.
❌ Unrecognized word in docs/guides/hello-world.ipynb: worlddddd


---------------

🙅 There are unrecognized words (see above). If the word is a not a typo, there are two ways to allowlist it:

1. Add a comment on a dedicated line to the first markdown cell in the Jupyter notebook.  (This cell should have the page title.) For example, add the following above the H1 heading: {/* cspell:ignore helloooooo */}

2. If you expect this word to appear in other files, add it to a dictionary. Add names to the `scripts/config/cspell/dictionaries/people.txt` file. Add scientific or quantum specific words to the `scripts/config/cspell/dictionaries/qiskit.txt` file. If it doesn't fit in either category, add it to the `words` section in `scripts/config/cspell/cSpell. json`. The word is not case-sensitive.

[Eric-Arellano]

@beckykd @abbycross review appreciated on the README and the error messages I put in the PR description.