Add header to qpy file for footnotes

[Eric-Arellano]

Qiskit/documentation#1987 adds proper support for footnotes to the docs. The docs will render much more clearly if we have an h2 to delineate where the footnotes start.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core
• @mtreinish
• @nkanazawa1989

[coveralls]

Pull Request Test Coverage Report for Build 10995274058

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.003%) to 88.802%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.02%
crates/qasm2/src/lex.rs	2	93.23%

Totals
Change from base Build 10960010824:	0.003%
Covered Lines:	73496
Relevant Lines:	82764

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💛 - Coveralls

[ElePT]

LGTM. #13202 should fix the QPY backwards compatibility test.