Global review

[sacha-l]

This PR is a general pass on content to address miscellaneous issues, including:

• Closing Feedback Submission - docs - Welcome to the Substrate community - /v3/contribute/style-guide #247 and Feedback Submission - docs - Submitting articles to receive a bounty - /v3/contribute/bounties #245
• Add a "warning this is WIP" message in Contribute sections
• Closing Exponent missing in weight chapter #233 and set_code link needs updated #213
• Closing Fix TODOs throughout #158: all TODOs were removed, the majority of them were in relation to Update parachain content  #239 or Parachain content #179
• Closing Why are SignedExtension part of the extrinsics doc? #156 : I moved the Signed Extension section out of "Extrinsics" and into "Transaction Pool"
• Fixing all headings that don't follow our new sentence case style convention

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[nuke-web3]

• Closing Fix TODOs throughout #158: all TODOs were removed, the majority of them were in relation to Update parachain content  #239 or Parachain content #179

Just wanted to check in on this - were the TODOs fixed in this PR, or just removed? If the latter, let's please highlight them in #179

[sacha-l]

@nukemandan : The TODOS for parachain how-to guides that were removed were in relation to adding Examples and References. There are still some that were left in (global search: FIXME). What this PR is saying in relation to closing the TODO issues (#158) is that #179 will handle the remaining fixes because all that remains is very parachain content specific. And it's already highlighted with the "FIXME TODO" or "TODO:FIXME" markdown notes ;)

[nuke-web3]

What #158 was about was keeping an open issue to ensure there were no lingering TODO:FIXME times left at all if I understood correctly - Perhaps a new tracking issue is needed for this that excludes anything that isn't covered in #179? Happy to do this if we need it then. I will add a note in 179

[sacha-l]

I don't think having 2 issues for the same thing is necessary. In this case, we already have issues open about completing parachain content. It turns out the only FIXMEs / TODOs we have outstanding are for that content AFAIK.