docs: Clarify develop and add manual-run commands

[bryphe-coder]

This adds a couple of missing pieces to the docs:

• More info about ./develop.sh
• Steps to run the built coder binary manually
  • Starting the server
  • Logging in
  • Creating a project

I attempted to keep them relatively agnostic so they wouldn't be out of date immediately when #422 is merged

[codecov]

Codecov Report

Merging #455 (cf05e97) into main (548b4f2) will increase coverage by 3.68%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #455      +/-   ##
==========================================
+ Coverage   64.23%   67.91%   +3.68%     
==========================================
  Files          72      157      +85     
  Lines         794     9222    +8428     
  Branches       73       73              
==========================================
+ Hits          510     6263    +5753     
- Misses        270     2331    +2061     
- Partials       14      628     +614     

Flag	Coverage Δ
unittest-go-ubuntu-latest	67.49% <ø> (?)
unittest-go-windows-2022	62.22% <ø> (?)
unittest-js	64.23% <ø> (ø)

Impacted Files	Coverage Δ
httpmw/workspaceparam.go	76.47% <0.00%> (ø)
coderd/coderd.go	96.18% <0.00%> (ø)
database/db.go	55.17% <0.00%> (ø)
agent/usershell/usershell_windows.go	0.00% <0.00%> (ø)
peerbroker/proxy.go	57.55% <0.00%> (ø)
coderd/workspaces.go	51.29% <0.00%> (ø)
peerbroker/dial.go	75.43% <0.00%> (ø)
database/pubsub_memory.go	94.87% <0.00%> (ø)
peer/conn.go	78.00% <0.00%> (ø)
cli/workspaces.go	100.00% <0.00%> (ø)
... and 75 more

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[greyscaled]

Nice!

[presleyp]

This is great, thanks!