Generate docs of v0.49.x

[t-kikuc]

What this PR does / why we need it:

by make release/docs version=v0.49.0

cf. https://github.com/pipe-cd/pipecd/blob/master/RELEASES.md#generate-document-for-new-version

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[Warashi]

👍🏻

[ffjlabo]

🚀

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 22.83%. Comparing base (b60d1bf) to head (13f608c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5222   +/-   ##
=======================================
  Coverage   22.83%   22.83%           
=======================================
  Files         419      419           
  Lines       45304    45304           
=======================================
  Hits        10344    10344           
  Misses      34165    34165           
  Partials      795      795           

Flag	Coverage Δ
	22.83% <ø> (ø)

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