built year level index.html files

Added `bin/all-years.sh` to run a command over all years.

Added `bin/gen-year-index.sh` to build an `index.html`
for a given year.

Added support code to build an `index.html`
for a given year.

Added `make gen_year_index` to to level Makefile
and related rules.

Using the above, added `index.html` files for all IOCCC years.

Updated `bin/README.md` according to the above.

Improved comments in `bin/all-run.sh`.

Exported TOPDIR in all `bin/` tools that use TOPDIR.

Removed incorrect lines in `bin/quick-readme2index.sh`
usage message.

Improve -v 3 printed important variables messages,
in order that important variables are exported,
from all `bin/` tools.

Rebuild all HTML pages via `make www`.

bin/README.md

@@ -30,6 +30,17 @@ bin/all-run.sh -v 3 bin/readme2index.sh -v 1
 ```
 
 
+### [all-years.sh](all-years.sh)
+
+Run a command on all IOCCC years.
+
+For example:
+
+```sh
+all-years.sh -v 1 bin/gen-year-index.sh -v 1
+```
+
+
 ### [gen-authors.sh](gen-authors.sh)
 
 Generate the top level `/authors.html` page.
@@ -52,6 +63,17 @@ bin/gen-location.sh -v 1
 ```
 
 
+### [gen-year-index.sh](gen-year-index.sh)
+
+Generate an `index.html` page for an given IOCCC year.
+
+Usage:
+
+```sh
+bin/gen-year-index.sh -v 1 2020
+```
+
+
 ### [gen-years.sh](gen-years.sh)
 
 Generate the top level `/years.html` page.
@@ -173,7 +195,7 @@ bin/all-run.sh bin/sort.gitignore.sh -v 1
 Print default substitutions.
 
 
-### [subst.entry-index..sh](subst.entry-index..sh)
+### [subst.entry-index.sh](subst.entry-index.sh)
 
 Print substitutions for a entry index.html.
 
@@ -183,6 +205,16 @@ Print substitutions for a entry index.html.
 Output substitutions for navbar on behalf of a entry.
 
 
+### [subst.year-index.sh](subst.year-index.sh)
+
+Print substitutions for a year level index.html.
+
+
+### [subst.year-navbar.awk](subst.year-navbar.awk)
+
+Output substitutions for navbar on behalf of a year level index.html.
+
+
 # <a name="how"></a>How IOCCC HTML content is built
 
 The [md2html.sh](md2html.sh) tool is the primary tool that