docs: include header and footer in manpages; move man to its own dir

docs/man/footer.5

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+.SH "AUTHORS"
+.PP
+neovim-flake contributors

docs/man/header.5

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+.TH "neovim-flake" "5" "01/01/1980" "neovim-flake"
+.\" disable hyphenation
+.nh
+.\" disable justification (adjust text to left margin only)
+.ad l
+.\" enable line breaks after slashes
+.cflags 4 /
+.SH "NAME"
+neovim-flake configuration specification
+.SH "OPTIONS"
+.PP
+You can use the following options to configure neovim-flake:
+.PP

docs/man/neovim-flake.1

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+.Dd January 1, 1980
+.Dt neovim-flake 1
+.Os neovim-flake
+.\" disable hyphenation
+.nh
+.\" disable justification (adjust text to left margin only)
+.ad l
+.\" enable line breaks after slashes
+.cflags 4 /
+.Sh NAME
+.Nm neovim-flake
+.Nd A highly modular, extensible and distro-agnostic Neovim distribution for Nix/NixOS.
+.
+.Sh BUGS
+.Pp
+Please report any bugs that you might encounter on the
+\m[blue]\fBproject issue tracker\fR\m[]\&.
+
+.Sh SEE ALSO
+.Pp
+\fBneovim-flake\fR(5)
+
+.Sh AUTHOR
+.Pp
+\fBneovim-flake contributors\fR
+.RS 4
+Author.
+.RE
+
+.Sh COPYRIGHT
+.br
+Copyright \(co 2023\(en2024 neovim-flake contributors
+.br