Keep the frontmatter when formatting the docs

[DavidBadura]

make docs parses every page with CommonMark and renders it again. Without front matter support --- became a thematic break and searchable: false together with the closing --- a heading. Both docs scripts now register the FrontMatterExtension of league/commonmark with a parser which keeps the raw YAML, so the front matter is written back byte for byte. The scripts are the same as in the event-sourcing repository now, the extension is part of league/commonmark, no new dependency is needed.

None of the pages on 2.1.x has front matter yet, but the upgrade guide on 3.0.x has, and every make docs broke it there.

[github-actions]

Hello 👋

Here are the most recent benchmark results, compared against 2.1.x. Time is the mode over the iterations in the its column, with the relative standard deviation in brackets.

StackHydrator

subject	revs	its	memory	base	current	diff
benchHydrate1Object	1000	5	1.855mb -21.23%	1.179μs (±1.91%)	1.213μs (±0.28%)	+2.85%
benchExtract1Object	1000	5	1.855mb -21.23%	1.378μs (±1.74%)	1.394μs (±2.32%)	+1.11%
benchHydrate1000Objects	3	5	1.855mb 0.00%	1.170ms (±3.52%)	1.158ms (±1.68%)	-1.03%
benchExtract1000Objects	3	5	1.855mb 0.00%	1.171ms (±1.89%)	1.131ms (±0.28%)	-3.43%

StackHydratorWithLazy

subject	revs	its	memory	base	current	diff
benchHydrate1Object	1000	5	1.855mb 0.00%	0.190μs (±8.14%)	0.189μs (±3.41%)	-0.48%
benchHydrate1ObjectTriggerInit	1000	5	1.855mb 0.00%	1.571μs (±1.71%)	1.513μs (±1.25%)	-3.67%
benchHydrate1000Objects	3	5	1.855mb 0.00%	182.309μs (±0.86%)	182.391μs (±2.45%)	+0.05%
benchHydrate1000ObjectsTriggerInit	3	5	1.855mb 0.00%	1.529ms (±1.37%)	1.543ms (±1.33%)	+0.86%

StackHydratorWithCryptography

subject	revs	its	memory	base	current	diff
benchHydrate1Object	1000	5	1.855mb 0.00%	1.870μs (±2.82%)	1.799μs (±1.85%)	-3.76%
benchExtract1Object	1000	5	1.855mb 0.00%	4.866μs (±3.60%)	4.652μs (±0.81%)	-4.41%
benchHydrate1000Objects	3	5	1.855mb 0.00%	1.756ms (±2.03%)	1.748ms (±0.67%)	-0.47%
benchExtract1000Objects	3	5	1.855mb 0.00%	4.396ms (±1.64%)	4.603ms (±0.48%)	+4.70%

This comment gets updated every time a new commit comes in.