docs: add FS storage to docs

[cbaker6]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Readme is missing FS adapter even though link to full docs has details about the FS adapter.

Related issue: #6768 and parse-community/docs#780

Approach

Add the FS adapter to the README and clean up this section to match the rest of the document.

TODOs before merging

• Add changes to documentation (guides, repository pages, in-code descriptions)
• A changelog entry is created automatically using the pull request title (do not manually add a changelog entry)

[parse-github-assistant]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov]

Codecov Report

Merging #7681 (70c4b76) into alpha (31960b5) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            alpha    #7681      +/-   ##
==========================================
- Coverage   93.89%   93.88%   -0.01%     
==========================================
  Files         183      183              
  Lines       13626    13626              
==========================================
- Hits        12794    12793       -1     
- Misses        832      833       +1     

Impacted Files	Coverage Δ
src/RestWrite.js	94.04% <0.00%> (-0.16%)	⬇️

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[mtrezza]

Looks good, is this ready for merge?

[cbaker6]

Yup, it’s ready

[parseplatformorg]

🎉 This change has been released in version 5.0.0-alpha.6

[parseplatformorg]

🎉 This change has been released in version 5.0.0-beta.10

[parseplatformorg]

🎉 This change has been released in version 5.1.0