docs: add file uploads

[Meschreiber]

Adds docs for the file uploads feature

Fixes #4629

Still to-do:

• [ ] Update list of enterprise features in monodocs - https://github.com/apollographql/docs/pull/773
• [ ] Update client docs to say that this is router supported + link to this doc
• [ ] Update https://www.apollographql.com/blog/file-upload-best-practices

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Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• [ ] Changes are compatible[^1]
• [ ] Documentation[^2] completed
• [ ] Performance impact assessed and acceptable
• Tests added and passing[^3]
  • [ ] Unit Tests
  • [ ] Integration Tests
  • [ ] Manual Tests

Exceptions

Note any exceptions here

Notes

[^1]: It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. [^2]: Configuration is an important part of many changes. Where applicable please try to document configuration examples. [^3]: Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions.

[github-actions[bot]]

@Meschreiber, please consider creating a changeset entry in /.changesets/. These instructions describe the process and tooling.

[router-perf[bot]]

CI performance tests

• [ ] const - Basic stress test that runs with a constant number of users
• [ ] xlarge-request - Stress test with 10 MB request payload
• [ ] events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• [ ] events - Stress test for events with a lot of users and deduplication ENABLED
• [ ] large-request - Stress test with a 1 MB request payload
• [ ] events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• [ ] reload - Reload test over a long period of time at a constant rate of users
• [ ] step - Basic stress test that steps up the number of users over time
• [ ] no-graphos - Basic stress test, no GraphOS.
• [ ] events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• [ ] events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• [ ] step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• [ ] events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• [ ] xxlarge-request - Stress test with 100 MB request payload