Adds a new "Getting Started on Testnet" documentation page addressing
developer questions about deploying to testnet. This was requested
because the docs currently guide developers to the local network but
don't explain how to use testnet.

The guide uses the **Sponsored FPC** at
`0x2ae02a54fd254586fd628ff46b71071bd8db32b63dc5d083f844f2c208a3923c` for
fee payment, so developers can get started without bridging Fee Juice
from L1.

The new page covers:
- Installing the correct testnet version of the CLI
- Registering the Sponsored FPC and using it for fee payment
- Creating and deploying accounts
- Deploying and interacting with contracts (Token example)
- Viewing transactions on Aztecscan
- Registering existing contracts in PXE
- Link to manual fee bridging docs for advanced users

Also updates:
- `sidebars-developer.js` — added testnet page between local network and
devnet
- `networks.md` — added Sponsored FPC address for testnet, updated links
and notes
- `how_to_pay_fees.md` — removed conditional blocks saying Sponsored FPC
isn't on testnet
- `getting_started_on_devnet.md` — removed stale testnet warning about
Sponsored FPC
- `getting_started_on_local_network.md` — cross-links to testnet guide

- [ ] Run `yarn start` in docs/ and verify the new page renders
correctly
- [ ] Verify sidebar shows the testnet page between local network and
devnet
- [ ] Verify cross-links from networks.md and local network page work
- [ ] Verify preprocessor macros (`#include_testnet_version`) resolve
correctly
- [ ] Confirm Sponsored FPC address
`0x2ae02a54fd254586fd628ff46b71071bd8db32b63dc5d083f844f2c208a3923c` is
correct and funded

Backport of #22366 to `v4-next`.

## Summary

Adds a "Getting Started on Testnet" guide and cross-links it from the
local network guide and the developer sidebar. Single commit, docs-only,
no code changes.

## Cherry-pick notes

`git cherry-pick -m 1 3b8d1f3` produced no conflict markers — only a
file-location warning because the PR added its versioned copy under
`docs/developer_versioned_docs/version-v4.2.0/` (the latest versioned
docs dir on `next`), and on this branch the equivalent directory is
`version-v4.1.0-rc.2/`. Git's rename detection mapped the new file to
the correct backport location; resolution was just `git add` +
`cherry-pick --continue`. This matches how prior backports (#22232,
#22193, etc.) have updated the v4.1.0-rc.2 versioned copies.

No conflict-resolution or build-fix commits were needed, so this is a
single clean commit rather than the 3-commit backport shape.

Analysis gist:
https://gist.github.com/AztecBot/1cc7e89de52ec1fdaef0b2194addf1af


ClaudeBox log: https://claudebox.work/s/90bb9caf79cca71d?run=1

BEGIN_COMMIT_OVERRIDE
fix(pxe): cap event filter toBlock to last synced block (#22573)
fix(pxe): round tx expiration timestamp to reduce precision (#22577)
fix: eliminate anvil watcher warp race and false success logs (#22584)
refactor: aztec new and init creating 2 crates (#20681)
test: aztec new scaffold works (#20711)
feat(cli): warning if contract crate has tests (#20723)
feat(cli): auto-recompiling when aztec test is run (#20729)
feat: aztec new supporting multiple contract crates (#21007)
feat: asserts that aztec dep version matches cli (#21245)
chore: backport aztec CLI improvements to v4-next (#22587)
feat: check noir release has nargo binaries before releasing (#22551)
chore: cache chainInfo in embeddedwallet (#22592)
fix: wrap external getCapsule in transactionAsync (#22595)
fix(pxe): throw clear error for invalid comparator in pick_notes
(#22585)
refactor(aztec-nr): rename conversion fns to encode_/decode_ naming
(#22576)
feat: infrastructure for testing `[new_contract_artfiacts,
old_aztec_stack]` (#22593)
chore: fix unnecessary and inconsistent side-effect counter increments
(#22245)
fix: update FaceID wallet redirects and strip anchors in redirect
validation (#22505)
docs: add getting started on testnet guide (#22366)
docs: add getting started on testnet guide (backport #22366) (#22619)
feat(aztec-nr): new BoundedVec emit private log APIs (#22064)
END_COMMIT_OVERRIDE