docs: rename 'connect to testnet' section

[Dhaiwat10]

As it turns out, we already have a page that goes over how one can connect to the Fuel testnet. This section was named as 'Connecting to external node' though. I have renamed it to 'Connecting to Testnet' via this PR.

Some ideas for further improvement

• The reason why the lack of these docs was mentioned in the feedback was probably the fact that they are a bit hidden under the 'Provider' section in the sidebar. Maybe we can move 'guides' like this one to a specific section? The 'Cookbook' section is a good candidate but cookbooks are usually reserved for rather advanced examples. Any thoughts?
• The example on the page itself demonstrates initializing a wallet on the testnet and does not actually interact with any contracts on the testnet or with the testnet node itself. Maybe we should update the example? Update the 'connecting to testnet' example #1690 (comment)

Closes #1548

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
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🟡	Functions	77.95%	1011/1297
🟢	Lines	86.89%	5903/6794

Test suite run success

1482 tests passing in 263 suites.

Report generated by 🧪jest coverage report action from 6a323cf

[danielbate]

Yeah I think these two should be handled separately. I definitely agree with renaming the page if that's what come from the feedback. Most other docs seem to reference it as a Testnet anyways.

The first point around hidden docs probably needs a bit more thought. We have vastly improved our documentation and added a lot more content, but it probably does need a more of a top-down approach now and see what we have and what's missing and whether the structure could be improved.

[Dhaiwat10]

@danielbate sounds fair. I'm gonna scope this PR to just renaming the section for now, then.

I'll file a separate issue to update the code in the example once beta-5 is officially up. We can then have the code example demonstrate how to interact with a live contract on the beta-5 testnet.

[Dhaiwat10]

Opened a discussion: #1690 (comment)

[github-actions]

Coverage Report:

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78.42%(+0%)	68.43%(+0%)	76.65%(+0%)	78.39%(+0%)

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Coverage values did not change👌.