docs: add scaling docs

[jemisonf]

Related to #5635. Adds a "scaling up" doc as discussed here.

Retained from previous docs:

• Monorepo scaling considerations is basically unchanged
• Scaling up is mostly unchanged, but renamed to "Settings Reference For Individual Components" (please feel free to bikeshed the name)

Added:

• As an introduction, a "Common Scaling Problems" section that should hopefully guide users towards how to address common issues. I am not positive the stuff listed there is actually the most common issues, so happy to take feedback on the specifics there
• A "Controller Sharding" with new content about how to manage a sharded controller -- includes some stuff we've adopted as best practices while working with controller shards that has not all been communicated back to the broader Argo community yet.
• A link to an empty metrics doc

The overall structure is a little different too -- I wanted to minimize the amount of time that someone with a typical problem would have to spend looking through the doc before they found something that would point them to the solution. Definitely open to feedback on how that actually looks, but I think it's a little more approachable than the previous structure. Where possible, I erred on the side of keeping content around rather than removing it.

Note on DCO:

If the DCO action in the integration test fails, one or more of your commits are not signed off. Please click on the Details link next to the DCO action for instructions on how to resolve this.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• Optional. My organization is added to USERS.md.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).

[jannfis]

Thank you @jemisonf! This is an awesome contribution. I could only nitpick on things like extraneous empty lines, but I won't :)

Much appreciated!