contributing index.md: add donate anchor

[plowsof]

the idea is to link the DONATE button/page @ https://ccs.getmonero.org/donate/ to instead link you to : (has translated strings.. working QR's for both Monero and Bitcoin) https://deploy-preview-2287--barolo-time-757cf9.netlify.app/get-started/contributing/#donate

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[nahuhh]

1. Unlike "getting started > contributing", the qr code on ccs.getmonero.org is clickable (has a proper monero:address uri)

2. The btc qr code on "getting started > contributing" is broken on dark theme browsers because it has a transparent border (needs to be white).

3. The broken xmr qr code is at the bottom of every page on ccs.getmonero.org, not just https://ccs.getmonero.org/donate

imo, makes more sense to ust swap out the broken ccs.getmonero.org xmr qr code image with the working qr code from here https://deploy-preview-2287--barolo-time-757cf9.netlify.app/img/donate-monero.png

And separately fix the btc qr code on "getting started"by removing transparency

[plowsof]

thanks for looking into this @nahuhh , i think my PR solves the broken ccs qr code @ https://repo.getmonero.org/monero-project/ccs-front/-/merge_requests/38

[nahuhh]

On gitlab, the png has 2 versions

left = broken + transparent(which also breaks it) right = correct