Manual work is almost never a good solution. Try this:
for PAGE in *.page
do
cat header.html "$PAGE" footer.html > “$PAGE.html”
done for PAGE in *.page
do
cat header.html "$PAGE" footer.html > "$PAGE.html"
done
HTH
EDIT: if you don't want the output file to be named "a.page.html" and instead it to be "a.html", replace the above cat invocation with:
cat header.html "$PAGE" footer.html > "${PAGE%.page}.html"
0 - https://man.freebsd.org/cgi/man.cgi?query=cat&apropos=0&sekt...
<!--# set var="pagetitle" value="Main page" -->
<!--# include file="00__header.html" -->
... main content here
<!--# include file="00__footer.html" -->