CMS5: Update frontend build doc

[maxime-rainville]

Follow up card to #1312

Acceptance critreria

• There's suitable doc for developers who want to do front end PRs.
• jQuery and Entwine aren't referred to as "legacy" or "deprecated"
• There's suitable doc for developers who want to update their module to be compatible with the new build stack.
• Docs which already exist are updated to reflect changes in Upgrade frontend build stack for CMS5 #1312
• The changelog reflects changes in Upgrade frontend build stack for CMS5 #1312

Notes

• This is not meant to be a comprehensive Doc card for our front end. It's just meant to give you the basic info you need to got our front end build working locally

PRs

• DOC Stop saying jQuery is legacy - it'll be around for a while. developer-docs#128
• DOC Update js docs developer-docs#130