Improve instructions for funded projects

I encountered a few post-funding steps that could benefit from extra information to improve the developer experience, especially for newbies. These are the bits that could use extra info/helpful links:

1. Tax implications for filling out the contractor form with NumFOCUS - there are a few options to consider here (at least these):
- US citizen/green card holder living in USA
- Non-US citizens living in USA (may be visa status also impacts here, e.g., may be international students are not eligible to work extra?)
- US citizen/green card holder living abroad
- Non-USA nationals living outside of USA

While everyone still needs to do their due diligence and ideally consult with a tax professional in their countries to understand tax implications, having some helpful web links or various scenarios would be handy. Hopefully, NumFOCUS has some material available already.

2. Break down the required steps to getting paid after project is funded:
- Sign contract with NumFOCUS
- Create personal account and possibly business account with OpenCollective
- (Do the work)
- Submit (monthly/quarterly) invoices to OpenCollective. This is two-step - once for the "text" invoice on OpenCollective but it requires an itemized pdf invoice as well. PDF invoice needs to contain the following:
- hours worked
- description of the work performed
- hourly rate
- billing date range
- Invoice Number

The expected granularity of the work description is unclear - are we expected to provide details for each chunk of work? For example, would each chunk of work need to have a separate work description and time log - e.g., "profile histogram code with astropy (1 hr)", or does a broad work description suffice - e.g., "performance optimisation (10 hrs)".

A separate thing that could be useful in the proposal phase would be different rubrics for time estimates based on the astropy package experience. As a software expert but an *astropy newbie*, I am surprised by how much time I have spent reading the infrastructure (e.g., CI jobs), documentation, and communication over GitHub / Slack.

Hopefully, this is helpful! My goal here is to see if we can improve the future contributor/developer experience - I am not fussed about myself.

Start by reviewing the existing post-funding guidance, NumFOCUS contractor-form material, and OpenCollective invoice requirements mentioned in the issue. Clarify tax scenarios, payment steps, invoice detail expectations, and proposal time-estimation guidance; done when these are documented with authoritative links and explicit examples for newcomers.

Written by the indexing model from the issue text.